How to be a classy teenage lady?

Im going to be a junior(11th grade) in highschool this September, and I want to give the impression that I am classy and respectable. So far, I haven't really put much care or effort into the way people see me. But I'm sick of girls who show off eeverything, act stupid and jut have no respect for themselves. I want to be the classy girl people look up to! What should I wear? What are key things to always remember? What should I never do? Thanks in advance for all your help (=