Hi! That’s awesome! :) I guess I would say wear something you feel comfortable in. In ballet class you normally go for a leotard, a pair of tights and ballet shoes. I tend to wear some leg warmers and a long pair of trousers at the beginning of class. Also a sweater or a hoodie. Because the body heats up faster and your muscles really will be warm. It is different from school to school what they want you to wear. If there is a dress-code you should stick to it. Otherwise feel free to wear ballet clothes but don’t be scared to go for some colour.
Be polity, listen to the teacher and do not use the mirror to check if you look good (just telling, when I teach classes there are way to many girls doing that). The mirror is helping you to correct your position and help you while training.
I wish you all the best and a lot of fun! Tell my how it was :)