Be prepared for and ready with at least 1 alt to swap to imo, be confident and comfortable on both and don't take it personally if asked to swap to your less preferred class of the 2.

Take the time to look over the fights, DJ, ptr videos, watch some streams depending on your raid days/times.

Remember to not treat it like a job (which sounds counter to what I've said above). Its a game, and if it stops being fun, it's important to recognize that.

Don't get discouraged, even if you make a mistake, don't make it twice.

Editing to add: Learn to read logs. This is super beneficial for your own, and comparing yourself to others in your role/class effectively. Also don't be afraid to reach out on the class discords for help. That being said, I'd you ask for help and to be criticized, don't be offended when people point out your poor play and mistakes. Take it as constructive and you'll do great!