Booking Shinkansen

Hello. I'm planning a trip to Tokyo and the Kansai area. My flights arrive in and depart from Tokyo, so I've decided a rail pass for the trip is the best option. Since I need to reach Kyoto on the day I land in Tokyo, is it possible to reserve seats on a Shinkansen online, or can it only be done at the train station? I'm visiting in July, and I'm worried there might not be available seats if I don't reserve them. Thanks