New player with a few questions.

[Niwashi]

you will want to get your thaumaturge, conjurer, and archer to lv.32 eventually for all the necessary skills at end-game. However, for now, simply get your Archer and Conjurer to lv.15 so you can class change.

Well, the thaumaturge only needs to get to 26 for all its cross-class skills. That's the one needed for either summoner or scholar.
Plus you'll eventually want either conjurer or archer to 34 depending on which job you're pursuing (both if you're using both jobs), but as a starting point to 15 for the job unlocks.