RESERVING SEATS DURING HIGH SEASON

Hi Clara,

You can start making seat reservations once you've arrived in Japan and can do so at any JR station or travel center.

Generally there are plenty of seats to go around and trains rarely sell-out but it is recommended to reserve seats 2-3 days in advance on busy days.

Also see:
http://www.jrpass.com/blogs/reserving-seats-boarding-the-bullet-train

Hope this helps,
Daniel