How should I ask her? Please!?

Okay, so I have been friends with this chick almost 2 years now. We go out on dates all the time... But as friends. And now she wants me to take her to her homecoming. And I am. So as u can see we are good friends. In fact she is prolly my best friend. But what I need to kno is how, and when to ask her to be my girlfriend. Not ask her on a date, cuz we've dine that, but to be my girlfriend. Ive never dine it before so I'm really like nervous and clueless... Help!