Stay relaxed, and attentive. Even if it's something you feel you know, you can always always learn something new, so don't let your mind wander.
Wear comfortable but not baggy jeans, a comfy shirt that won't get in the way of your arms, and boots with a slight heel. If you don't have any, hiking boots are ok (not recommended though because the traction can prevent you from taking your foot out of the stirrup quickly if you need to). Sneakers are not recommended for the same reason.
Hmm. If you don't have a riding helmet, call the barn you're taking lessons at and ask if they provide helmets. Some do, some don't.
And have FUN!! Don't get stressed out if you can't do something perfectly right away. That's why you're there, to learn. Lessons are fun and you get out of them what you put into them, so keep a great attitude and smile and you'll have a blast :)