Be upfront and confident. Don't be intimidated or cocky. Show her that you would really like to be with her. If you have never talked to her before find something to compliment her on or ask about something she is wearing. This should strike up a conversation. If you already know her be sure to stay out of the "friend zone". Don't let her think of you as one of her "girlfriends". Don't be too overbearing. Give her her space. If she likes you she will give subtle cues--Always trying to look her best around you, leaning in towards you while having a conversation, looking into your eyes, blushes or smiles easily, touches your arm or other places while talking, gives flirty compliments, and so on.