Provoke is the most important crossclass in the game, imo (no job/role depends on a crossclass ability more than this), the sooner you get it, the better.

I've met good tanks that think their bad and bad tanks that think they're good. For the learning process just keep in mind your priorities. Manage Aggro > Survival > DPS and Support. It helps if you can run with a healer-friend once in a while and they can let you know how it feels as a healer to keep you up, but for aggro (your top priority) I think you can sort of tell on your own.