Let your prospective partner know ahead of time if you don't know the dance. It's bad form to turn down a request to dance, then accept another offer for the same dance. Eye contact with your partner is good, penetrating glares and leers are bad. Know the local protocol. While folkdancing is a very social event, some fanatics really show up mostly to dance.
Good hygiene is always in style.
Learn the names of the dances you like and request them. The instructor's equipment is often expensive and breakable, and is not public property. Offer suggestions, comments, likes, and dislikes. Constructive feedback is better, but be sure to make your feelings known. Don't get discouraged, try stuff you don't know, smile once in a while, and have fun.