Always watch a guide before you go into practice. Unless you are joining a blind prog group, don't go in blind because you'll single handedly cause disbands. If you can comprehend the guide, you'll already be better than 80% of people you meet in practice parties. By the end of your first day you'll likely be ready for clear groups, I've never been in more than two practice groups before attempting a clear. You'd be surprised how quickly you can progress in this game once you get out of the initial starting blocks and beating PF anxiety.

I've also always recommended that people play something easy for their first clear, like a healer, a physical ranged, SAM, SMN etc. Something where you can look at mechanics and not worry about positionals and tracking buffs/debuffs, then come back to a main that has more complex rotations after you have your clear. Like I've always generally cleared first as AST and then gone back to something like Monk or Black Mage.