if you really like a girl, do not hesitate first. Just come up to her and always in her eyes. Smile and tell her that she has got a beautiful dress and you can't take your eyes from her. Then invite her to dance. I suppose she won't refuse as she'll see a gentleman beside her. Girls like compliments but you should be sincere and open. After that you'll see that she wants to dance with you again and again. Good luck! ;-)