Pick a dress that is appropriate for your school. Most schools have some sort of dress code that will dictate how revealing your prom dress can be. If you automatically gravitate toward styles that are modest and classy, this probably won't be an issue for you. On the other hand, if you tend to be someone who likes showing off a lot of skin, you should check into the dress code restrictions to find out what your school will let you get away with.
Prom dresses can either be long or short. If going for a long dress, make sure that the hem works for your height. If opting for a short dress, go for one that is no shorter than mid-thigh length. Any hemline shorter than that tends to drain all sense of elegance out of a dress.