You can book it now. Sometimes, it is a good idea not to wait until the last moment because some trains have a limited quota for Interrail reservations (mostly in/to/from France) or are just fully booked.

Okay, and for train without reservation, we just have to go in the train, and show our interrail pass to the controller ? Or we have to call the train before take it to make sure there are seat for us in the train ? And if there are no more seat in the train, what we do?