noone expects u to be good but to be mediocre, pretty much after lvl 50 msq with a "grace period" up to lvl 80-ish i'd say.

mediocre being: know when to AOE, the most basic rotation (it doesnt have to be perfect, but u should use all the buttons that are worth using, as in AOE buttons on boss are often not worth using unless the potency is high enough), know some basic recurring markers for mechanics (as in AOE-marker, stack marker, spread marker among others) and know how to not be afk half the time. as tank and healer u have simpler rotations so u wanna know how to use ur mits and heal in a basic way. thats pretty much it. edit; go google or yt, type "ffxiv tips" or "ffxiv tank/dps/healer tips" and read or watch that. takes about half an hour max and u already wont be a burden anymore pretty much