Use a reliable planner, e.g. the DB website or DB Navigator app to plan your journey.

Check this thread on the best places to get reservations:

If you still have questions on reservations, then please mention route, date and departure time of the trains you'd like to book.

If it's just one return trip you're doing, then normal tickets (book via bahn.de) will probably be cheaper.

Profile says you study-hence live for now, in NL. The 2-hourly EC trains Ams-B are with only optional RES-but may be advisable on busy days around Xmas (can do easily online via bahn.com-4,50).

IF you want-like so many youngees, want to travel overnite-even that can do-take eve train to Dusseldorf and change there to ICE (seats only, RES optional) to B-arr early next morning.

The direct train B-Krk (I was there some 2-3 weeks ago) has mandatory RES-and this can effectively only be done at DB-counters for now-and probably also not that long advance. YOu have more connections via Poznan (direct train to WAW), IN side PL all IC-trains by PKP have mandatory RES (1 PLN per ride=22 cts).

IF you just want to visit Krk and are not too much concerned about going on rails only: fly with Wizzair from EINdhoven.