Flip flops to wear to the mat and in the shower.
If there's a shower at the gym, that's a godsend, shower right after or as soon as you get home, everything you wore on mat goes into the washer then hang to dry and maybe will be dry in time for tomorrow's To avoid ringworm, which runs rampant in gyms, shower before sweat dries on you if at all possible or asap. Don't let your sweat soaked gear sit. Just like with yourself, wash asap.
Speaking of showers, just have good hygiene, you're going to be in close close contact with others, paired off with a training partner, don't eat after a couple hours prior, keep fingernail and toenail clippers in your gym bag , you'll need to keep those cut short.
Try to resist temptation to ask a million questions during instruction but at same time don't let yourself get lost, speak up here and there as needed to stay on track.
Always stay aware that your goal in addition to exercise and fun is to learn and help your training partner learn as well. Everyone will be at a different place in their journey.