Many high-speed and night trains can be booked up to 3 months in advance. The exact booking period depends on the train company. You can find more information about specific trains on Interrail's train pages.
Book as early as possible, especially in summer and around Christmas. Popular trains often sell out before the day of travel. If you need a paper ticket, allow extra time for it to arrive by post.
Train fully booked? Use the Planner to find another train. To see only trains that don't need a seat reservation, tap Filters and turn on No seat reservations required. Tips to avoid seat reservations