When you take the Shinkansen or the limited express train, you may have to book a seat because some trains require you a seat reservation. Though some limited express trains have some non-reserved cars, you should book a reserved seat if you can.
Especially if you use JR Pass, Hokkaido Rail Pass, JR East Pass, Tokyo Wide Pass, Sanyo San’in Area Pass and Kyushu Rail Pass, these passes cover a reserved seat fee. Use this benefit to ride a train with a peace of mind.
I also strongly recommend you to secure a seat if you travel in the peak seasons, such as late April to the first week of May (a.k.a. Golden Week), August 10 to 20 (It is called “Obon” which is a summer break.) and the year-end/New Years holiday (December 29th to the first week of January.)
I would like to explain you how to book a reserved seat in this post
Before you make a reservation
If you intend to use Japan Rail Pass (a.k.a. JR Pass), JR East Pass, Tokyo Wide Pass, Sanyo Sanin Area Pass, Takayama Hokuriku Area Pass, Hokkaido Rail Pass and Kyushu Rail Pass, you MUST turn in your exchange order to the actual Pass before booking your seat. You have to show your pass when you make a reservation. Exchange order does not work to book a train seat. Once you turned in, you can go ahead to make a reservation at any reservation windows and anytime. But if you use JR Easts’ online reservation, you can book a seat in advance before exchanging an actual pass. I will explain about this online reservation site later in this post.
Explaination by movie
You can watch the movie below which explains you about the booking in the station:
If you buy the JR Pass on the official reservation site, you can book a seat online. I explain how to use this site in the movie below:
I explain how to use JR East online booking step by step in the movie below:
Where you can book
First of all, I would like to tell you how to book a seat at the station. I will explain you about online booking later in this post.
You can book a seat at the ticket reservation office that is called “Midori no Madoguchi (みどりの窓口)”. You can find “Midori no Madoguchi” very easily. You will find a big green sign easily like the images above and below.
“Midori no Madoguchi” is located at most of major JR stations. If you are in major cities, such as Tokyo, Osaka, Kyoto, Yokohama, Fukuoka, Sapporo, you cannot miss “Midori no Madoguchi”. 99% of JR stations in these cities have “Midori no Madoguchi”. Some major stations have a few Midori no Madoguchi.
The signage is different in JR Central area.
If you travel in JR Central (JR東海) area, you cannot find Midori no Madoguchi. Because “Midori no Madoguchi” is JR East’s trademark and JR Central avoids to use it. In JR Central area, such as Nagoya, Mishima, Shizuoka, Gifu, Takayama, it is shown as “Shinkansen and JR Line Tickets (新幹線・JR線きっぷうりば)” like image below.
JR Central operates the Tokaido Shinkansen between Tokyo and Shin-Osaka. JR Central has a ticket office outside JR Central operation area, such as Tokyo, Shin-Yokohama, Kyoto and Shin-Osaka. In these stations, you can find two different kinds of the ticket office, “Midori no Madoguchi” and “Shinkansen and JR Line Tickets”. But you can use either ticket office to book any JR train tickets. This difference does not matter.
What kind of information you need
Please make sure the following information before you make a reservation:
- Date and time
- Train name and number
This is not required. But if you want to take the specific train, please tell the train name and number. It is much easier to make a reservation for the staff and you can save your time. All Shinkansen trains and most limited express trains have the combination of the name and number. e.g. “Hikari 246”. - Departure and Arrival Stations
- Smoking or Non Smoking
Smoking seat is still available on a few trains. If you really want to have a seat in the smoking car, you may ask a staff to look for. If you do not mention, usually you will be assigned a seat on non smoking car. If a seat is available on only smoking car, usually a staff let you know and make sure that you accept it or not. - Passport
If you use JR Pass or other JR passes, you always have to carry your passport. You may be asked to show it by a station officer or a conductor when they want to check it.
When you can start booking
You can make a reservation anytime within one month prior the departure date. The ticket is on sale one month prior to departure date at 10:00am
e.g.
Departure date — August 23 at 10:00 am. the Shinkansen from Tokyo to Shin-Osaka
Ticket is on sale at 10:00 am on July 23.
e.g.
Departure date — August 24 at 00:34 am. Sunrise Seto from Osaka to Tokyo
Ticket is on sale at 10:00 am on July 23, not July 24. Because this train originally depart from Takamatsu at 21:26 on August 23. Overnight trains are very few in Japan currently. But if you book a seat on overnight booking, please check the departure date at the original departure station.
The reserve seat on most trains can be reserved at Midori no Madoguchi only. So you have to be in Japan when you book. Online booking is still very limited.
Online booking
Japan Rail Pass official site
If you buy the JR Pass at the official site, you can make a booking through this website.
Japan Rail Pass official reservation website
You can book a seat on most JR trains. However the price of JR pass is higher than the prices at the authorized dealers. If you buy the JR pass at the authorized dealer, you cannot book a seat on this site.
JR East Train Reservation
This online reservation system is operated by JR East. Unfortunately this JR East Shinkansen Reservation is only English online reservation for the traveler who use JR Pass, Hokkaido Rail pass and JR East Passes. If you intend to use a regular ticket, you can use this site too.
This site offers a seat reservation for the trains which run in JR East and JR Hokkaido areas. So you CANNOT make a reservation for the Tokaido-Sanyo Shinkansen (Tokyo-Kyoto or Osaka) and other limited express trains which does not run in JR East and JR Hokkaido area. And also there are some restrictions. The biggest restriction is the ticket pick up. You must pick up a ticket one day before the travel date. You cannot pick up a ticket on the day of travel. Please read Guide (FAQ) before you make a booking.
Tokaido-Sanyo Shinkansen Reservation App “EX”
This app is available in a few countries only. And also this app is available for a regular ticket user only. You cannot use this app to book a seat for JR Pass or other rail passes. However any rail passes do not meet your trip plan and this app is available in your country, this is very convenient. You can book it in advance from your smartphone.
JR West Online Train Reservation
JR West has launched the online reservation site. However this online booking system does not cover any train pass user. You can use this online reservation as the regular ticket user only.
If you use a regular ticket, this online site may be a good choice. This online booking covers most of JR trains.
Eki-net (えきねっと)
I have received a comment from one of my readers, Tak. He mentioned me that we can use this Japanese online reservation system even for Japan Rail Pass or other travel passes users. This online reservation is in Japanese. So unfortunately this is not available for all of you. However some of my readers asked me that we can use a friend in Japan to make a reservation for Japan Rail Pass user in outside of Japan. You can do it with your friend in Japan and use this site.
This reservation system covers all over Japan. You can book for the Shinkansen between Tokyo and Shin-Osaka which is not available on JR East Train Reservation.
How to reserve a ticket from outside Japan in advance
As you saw, online reservation does not cover all trains. I really wish Japan Railway have full online reservation system.
If you want to secure your seat before departure, the local travel agent may be able to help you. This is only way to reserve a ticket before you depart from your country.
One of my frequent visitors, James in Australia, left a comment. He did this way many years ago. But I would like to share this information:
*JTB actually charge MORE than $55 Australian dollar($56 US dollar March 2011 currency) per ticket, in fact JTB charge $75 Australian dollar(US$76) per ticket.
The $75 came from $55 service fee plus $20 courier fee to deliver your ticket to your hotel in Japan, $55+$20=$75 Funny my ticket for SL-Yamaguchi express is only $60 therefore altogether I need to pay $135 to get SL-Yamaguchi express ticket!
So, all I can say is, if you want to get a JR train ticket before you get to Japan, make sure you are ok with paying at least $75( $76 US dollar), it helps if you really need to get on a particular train that is very hard to book or if you are travelling during peak season. But if you are not in a hurry, travelling in leisure pace across Japan, perhaps think twice.
I hope is worthy many people told me SL-Yamaguchi express is the best steam train experience ever in Japan.
Unfortunately this is only way to do it. You can find Japan Rail Pass authorized agent at JR Pass official site. They may be able to reserve a ticket for you too. But some of the agent do not want to reserve your seat. They want to book whole things, like hotel, air ticket, attraction tickets. Because it is too small profit to make a reservation for only train ticket.
Comments
Hi Takeshi,
I am going to Japan this 23rd October arriving at KIX. I am going to Kagoshima and leaving KIX on 27th October.. I understand that JR has this “Sanyo-Shikoku-All Kyushu” tickets. My question is, is this ticket available to purchase online? If they do, where can I find the link? If they don’t have links to purchase online, can I purchase the ticket at KIX? I read in the website (http://www.westjr.co.jp/global/en/travel-information/pass/all_kyushu/) stating that in order to buy the rail pass in Japan at KIX, I will need an e-ticket because it says “an e-ticket must be printed out” means I have to print out the train ticket (that I bought online?) to exchange with the rail pass? Or does the statement of “e-ticket” is referring to e-ticket of my return flight? In order to get the best fare, I suppose this “Sanyo-Shikoku-All Kyushu” is the best option right now, am I right?
Hope you can give me some advise on this matter.. Thanks in advance..
Hi Danial,
E-ticket means flight ticket, Because JR staff needs to know that you are temporary visitor. That’s why they check your status and e-ticket.
It’s not available to purchase online. But it’s completely no problem to purchase this ticket at KIX. It’s a perfect choice for your trip, I’m sure.
Cheers,
Takeshi / JP Rail
Thank you so much Takeshi.. That explanation clears a lot of curiosity in my head.. 😀
No problem, Daniel.
Cheers,
Takehsi / JP Rail
Hi Takeshi,
I’m a little off topic here, I don’t have JP rail pass. But I wanted to know about online reservations for busses and trains. I’m travelling this November from Kyoto to Kanazawa. Have 2 options here:
1. Take a thunderbird (what’s a Japanese name?) train: 7000 yen for a reserved seat. Or
2. Take a bus from Kyoto to Kanazawa. I read that there are overnight busses, but couldn’t find info in English (which bus, from where in Kyoto to take the bus, online reservation). There are also busses from Kyoto station, how can I reserve a sit online? (or not possible?). And there is a Willer express bus departing from highway, but I don’t know how to get to the bus stop (which as I understand is outside Kyoto?).
3. And I to go from Kanazawa to Osaka for my flight. I can take a train, but I’m on a budget. Can you advise which bus to take? Are there overnight busses or early morning (5 -6 am) busses?
Would really appreciate your help,
Aizhan
Hi Aizhan,
There are many many bus services in Japan. But as you tried and couldn’t find it, English site is very very rare. I have never seen other than Willer. Even I am overwhelmed to find it because too many.
As long as I know JR Nishinihon bus operate daytime and overnight bus between Osaka/Kyoto – Kanazawa. This is Japanese link but I put it here just in case.
http://www.nishinihonjrbus.co.jp/
Even if you use Willer, I don’t think Kyoto-Kanazawa does not run.
http://willerexpress.com/x/bus/dynamic/3/en/html/pc/route/list/hokuriku.php
So it’s impossible to book it without Japanese in advance. If you want to try to book it on the spot, you can get JR Bus counter at Karasuma exit (central exit) of Kyoto station. You may be able to book it on the spot.
Unfortunately that’s all I can do for you.
Cheers,
Takeshi / JP Rail
Thanks Takeshi!
It is really helpful.
Can you write down the Kanazawa (city and prefecture, train station) and Kyoto in Japanese? I want to try to book a ticket through the website you provided.
Hi Aizhan,
Sorry. But I don’t translate anything in this site. Once I do this for even a few words, I have to do same service for everybody.
Please understand this matter.
Cheers,
Takeshi / JP Rail
It isn’t difficult to open up Google maps and look for kanaZawa. Kyoto is dead easy. Even google translate will get you the names of these large major stations. Takeshi provides an excellent service in this website.
Wonderful information, here.
We want to book a trip on the Yufuin no mori on November 16th, 2014. Can we use eki-net to reserve the panorama window at the front of the train on October 16th, or are these green seats?
We won’t arrive in Japan until the 5th of November, but would like to specifically reserve that train as soon as possible.
Hi Armstrongest,
You can book it at Eki net. But unfortunately you can’t assign the seat. There are some choices, like window side, both end side. If you choose “end side”, you may be able to get front row, but not guaranteed.
There is no green seats either.
Cheers,
Takeshi / JP Rail
So, just to be clear… I can’t reserve a specific seat on eKi-net. Can I do this in person in Japan?
Or, can I reserve a seat in ekinet and when I arrive in Japan change the seat or is this discouraged ? My wife is Japanese, so that might help.
Hi Armstrongest,
No, you can’t at Eki net. You can do it in Japan, but as you guess it may be sold.
You can book some seat at Eki net anyway. You can change when you arrive in Japan. But it may be same story. Front row seat is very popular. The chance to get the seat after you arrive in Japan is very very slim.
If you really want to book front row seat, why don’t you ask your wife to call Odekake net. You have to pay but you can book a specific seat. You also can pick up a ticket at JR Kyushu’s station.
Cheers,
Takeshi / JP Rail
hello!
how can i book an overnight bus from tokyo to kyoto on january the 1st 2015?
thanks so much!
Marco
Hi Marco,
I think only Willer Express has English site.
https://willerexpress.com/en/
There are so many bus companies in Japan. But mostly Japanese site only.
Cheers,
Takeshi / JP Rail
hello
i am travelling from tokyo to kyoto then from kyoto to osaka. whilst in osaka i would like to travel to hiroshima and nagasaki and then back to osaka. what you suggest i do as far as buying a rail pass to get to these places…? i have not purchased a JR pass. will i need to reserve this online before i arrive, or can i purchase when in osaka…?
thanks!
Hi cb,
Can you purchase JR Pass. JR pass is the best choice for your trip. There is no other deals that covers all part of your trip.
Online reservation cannot accept to book for your trip because it is not JR east area. You have to book your ticket after you arrive in Japan anyway.
Cheers,
Takeshi / JP Rail
Hi,
This is Jie again. If we arrive at Kansai Airport around 1300 on 13september, is there any chance for us to buy any overnight train tickets to Hakodate or Sapporo the same day without any reservation? We need to arrive at Hakodate or Sapporo on 14september.
Where can we buy the railway tickets at the Airport? We will travel with JL pass.
Many thanks!
Hi Jie,
There is only one way to get Sapporo or Hakodate next day by overnight train. That’s Hokutosei.
https://jprail.com/trains/sort-by-type/overnight-trains/overnight-express-train-hokutosei.html
As you saw the link above, this train has berth and compartment only. You have to book it before taking this train. You can book it at JR Kansai Airport station. If you intend to exchange JR Pass at Kansai airport station, you can book it too.
Usually B berth is not very difficult to book. But nobody guarantee it. It may be sold out. In case of it, you have to find accommodation somewhere on the way to Hakodate. And also JR Pass does not cover fully. You have to add accommodation fee, express surcharge and non JR lines fare. You can find the amount at the link above. This train departs from Tokyo. You have to move to Tokyo from Kansai airport on arrival day. It will be very tight schedule to catch this train.
So I don’t recommend to use this. The schedule is very tight and many extra fares. If you need to be Hakodate next day, just take Shinkansen as far as you you can. If you can take 15:16 departure Haruka express at Kansai airport, you can get Aomori on arrival day around 23:00. If you take the earliest train from Aomori (at 6:52 Hakucho 71), you can get Hakodate at 8:58. If you take this train transfers, you don’t need to pay any extra on top of JR Pass.
You can find the hotel in Aomori much cheaper than Hokutosei accommodation fee.
Cheers,
Takeshi / JP Rail
Dear Takeshi,
Thank you so much for your useful information.
I think I will have to give up my plan to travel to Hakodate/Sapporo by train It is too stressful to find the same day train connection when we arrive. Probably we can try our luck on the way back home from Sapporo to Osaka on 20Sep instead!
Have a nice day!
/Jie
Hi Takeshisan,
Sorry, I have to ask you for one more question. If we travel on 13Sep from Kansai Airport to Sapporo instead by JL train, what is the best option that you can recommend? The earliest time that we can leave from Kansai Airport is around 14-15.
Thanks!
Jie
Hi Jie,
Sorry, but I don’t understand what you mean. What does 135ep means? (Time?) Are you going to take JL (Japan Airline) flight to Sapporo?
Please make sure what you need to know.
Cheers,
Takeshi / JP Rail
Dear Takeshi,
I am sorry that I did not make myself clear.
If we travel on 13th of September from Kansai Airport to Sapporo by Japan Railway, what is the best option that you can recommend?
The earliest time that we can leave from Kansai Airport will be around 1400-1500.
Thanks!
Best regards,
Jie
Hi Jie,
You can’t get Sapporo on arrival day by train. It’s too late to arrive to go to Sapporo from Osaka.
I think you can catch 15:16 departure Haruka to Shin-Osaka.
15:16 Kansai airport (Haruka 24)
16:05 Shin-Osaka
16:16 Shin-Osaka (Hikari 528)
19:10 Tokyo
19:20 Tokyo (Hayabusa 33)
22:30 Shin-Aomori
22:40 Shin-Aomori (Local train)
22:46 Aomori
So Aomori is the nearest place where you can go on arrival day. Hokkaido is quite far from Osaka.
But I don’t recommend to take the train for 7 hours after you arrive. Please think about staying Osaka or Tokyo on arrival day.
Cheers,
Takeshi / JP Rail
Thank you! How about if we take the airplane to Ibaraki Airport in Tokyo around 12 oclock in the noon time on 13th of September?
Will it be easier for us to travel to Sapporo from Ibaraki than Kansai Airport? We can exchange the Japan rail pass at Tokyo Station,right?
Best regards,
Jie
Hi Jie,
Ibaraki airport is much closer. But there are two issues.
If you get Tokyo station from Ibaraki airport, direct bus service by Kanto Bus is the easiest. (13:50 -> 16:20) But reservation is required to take this bus and there is only Japanese site available.
http://www.kantetsu.co.jp/reserve/reserve.cgi?lang=ja&route_sel=1&route_cd=001
If there are seats available, you can take this bus without reservation. I have no idea how busy this bus is.
If you can make this bus, you will be Tokyo station 3 hours early than arriving KIX. However it’s still late to get Hokkaido. You have to take 16:20 departure Shinkansen at Tokyo station. This is the last Shinkansen to connect to the train to Hakodate.
https://jprail.com/travel-informations/basic-informations/timetable-and-reservation/hyperdia-perfect-guide-train-timetable-and-route-search-engine.html
There is only one way to get Tokyo station by 16:20. Take the bus from Ibaraki airport to JR Ishioka station.
http://ibaraki-airport.net/en/access/train_bus/tbto_ishioka.html
If you can take 13:10 dep bus, you can make Tokyo at 15:18. (You can find this train schedule at Hyperdia too.)
This is only way to get Hakodate. You have to transfer several times and take local bus. It might be stressful.
If you can book domestic flight from KIX to Sapporo, it is the easiest. There are several flight from KIX to Sapporo (New Chitose airport) around 16:00.
3:50 pm → 5:45 pm Peach Aviation 107
4:10 pm → 6:00 pm JAL 2507
5:45 pm → 7:40 pm Peach Aviation 109
I don’t know about your budget. But this is the simplest way.
Cheers,
Takeshi / JP Rail
Dear Takeshi,
Thanks again for your detailed explaination:-)
I am going to travel with my 70 years old pappa so it will be quite stressful to make the same day connection as you said. We can take the flight to Hokkaido with Peach Airlines from KIX but we really want to check the Japanese landscape out by train….
I need to think it over again. Hope you do not mind if you write to you several questions again:-)
Best regards,
Jie
Hi Jie,
I like train trip more than flight too. But most of your trip on arrival day in the evening. You can’t see it because of dark. Shikansen trip from Osaka to Tokyo is very scenic. You can see Mt. Fuji, Lake Hamana and some more. But Tokyo to Aomori is not much because there are so many tunnels.
Please think about your plan again. And if you have some more questions, contact me.
Cheers,
Takeshi / JP Rail
Sorry for the troublesome! But I searched many times from HyperDia and I could not find this option on 13September that you copied and pasted here before:-( The earliest time to arrive at Aomori is 14September at 0848 when I checked from HyperDia. Can you please kindly advise? Thanks!
15:16 Kansai airport (Haruka 24)
16:05 Shin-Osaka
16:16 Shin-Osaka (Hikari 528)
19:10 Tokyo
19:20 Tokyo (Hayabusa 33)
22:30 Shin-Aomori
22:40 Shin-Aomori (Local train)
22:46 Aomori
Dear Takeshi,
Is it possible for me to reserve the seats for the same day leavning train after I have arrived in Kansai Airport and exchanged my JR pass?
Thanks again! 🙂
Hi Jie,
I can’t guarantee but it will be okay. Haruka and Shikansen are very easy to book.
Cheers,
Takeshi / JP Rail
Dear Takeshisan,
How about green seats reservation on Hayabusa train on 13September between Tokyo and Aomori on 13Sep?
Will it be very difficult to reserve the same day when we have arrived?
I know it is difficult to forecast the seat availability, but what would you say since you have so many experience with japan train:-) 🙂
Sorry for bothering you so much!
Best regards,
Jie
Hi Jie,
You can find the availability at the link below:
http://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/common/Menu/Menu.aspx
You can’t book it at this site because you have to pick up a ticket day before and at the station in JR East area.
Cheers,
Takeshi / JP Rail
Hi Takeshi,
As mentioned earlier I would like to reserve seats on the Japanese version of eki-net with the JP Rail Pass.
Do you think that I could use the address and phone number of the hotel to register myself?
Thank you for your answer!
Best regards,
Tom
Hi Tom,
Technically you can’t.
But it may be okay because I think nobody will check it.
Cheers,
Takeshi / JP Rail
Hi,
First of all, thank you very much for your extensive explanation! It helped a lot with planning my trip this August.
As I am traveling in one of the most busy month through Japan, I would like to make reservations in combination with the JP Rail Pass.
I read that this would be possible on Eki.net. How can I make this reservation with the usage of the JP Rail Pass?
Thank you in advance for your help!
Tom
Hi Tom,
If you can use Eki net, just use it by same way as regular booking. It will show you the fare and surcharge that will be charged to your credit card. But once you exchange JR pass and show the booking by Eki net, it won’t be charged. I always use this way when I go to Japan. I have never been charged to my credit card.
I recommend you to print out your booking list on eki net. It will make the procedure be faster and easier.
Cheers,
Takeshi / JP Rail
Hello!
My husband and I will fly into Narita on August 3, 2014, at approximately 1730. We know that we need to take a train to Tokyo Station, where we will get a train (JR Joetsu Shinkansen) to Echigo-Yuzawa, than change to the Hakutaka limited express to Takaoka, where our son will meet us. Do we need to make reservations for any or all parts of this journey? It appears that I cannot make reservations for the Narita to Tokyo Station portion until we land at Narita. Since I don’t know precisely when we will arrive at Tokyo Station (in case of delays with flights, baggage, etc.), I am not sure when to book a reservation for the Tokyo Station to Echigo-Yuzawa portion. I wonder if it is necessary on this day or at this time of day. Thank you in advance for your advice.
Hi Pam,
JR East has online reservation for Narita Express, Joetsu Shinkansen and Hakutaka. But you can’t use this because you have to pick up your ticket by 21:00 the day before you travel. You will continue travel to Takaoka right after you arrive in Narita. So you have to make a booking for all trains on the spot. You can make all booking at Narita airport station. But the problem is schedule. I checked the timetable. Your schedule is super tight. The last connection to Takaoka is the following:
Narita Express #46 Narita Airport 18:48 – Tokyo 19:49
Joetsu Shinkansen “Toki #347” Tokyo 20:12 – Echigo-Yuzawa 21:20
Limited Express “Hakutaka 26” Echigo-Yuzawa 21:30 – Takaoka 23:38
So you have to complete custom, immigration, baggage pick up and reservation in 1 hour 18 minutes.
It is not impossible as long as your flight will arrive on time. I have done it in 45 minutes. If you do this, you have to be hurry to get off the airplane, get custom and immigration as soon as possible. They work very quickly. As long as your document is okay, it takes lees than 10 minutes. Baggage will come off with 30 minutes after flight arrival.
And then go underground to get station and find JR East Travel Service Center. Please see the following link about arrival in Narita.
https://jprail.com/sample-itineraries/airport-access/narita/trip-to-tohoku-and-nagano-in-2013-winter-part-1-from-narita-airport-to-ueno.html
Hope you will make it!
Cheers,
Takeshi / JP Rail
Thank you very much for the information regarding trains to Takaoka from Narita. I rechecked our arrival time at Narita, and it is 1630, not 1730, so that does give us an extra hour. Pam
Hi Pam,
That’s good. Now I’m sure you can catch the last train to Takaoka.
Enjoy your trip!
Cheers,
Takeshi / JP Rail
We will be arriving at Narita on October 5, 2014. Can we book a train reservation on line from Narita to Yokohama (where we will board our cruise ship).
If not how do we proceed from terminal 1 to the train station?
Hi Huguette,
You can book it online.
https://jprail.com/travel-informations/basic-informations/timetable-and-reservation/how-to-reserve-seat.html
But I don’t think you need to do. There are many runs of Narita Express to Yokohama. It can be done at the Narita airport station. The station is underground of terminal building. There are lots of signage. You can’t miss it.
Cheers,
Takeshi / JP Rail