You need to psyche yourself up in front of a mirror, preferably when you know she is close, and then give yourself an ultimatum. Don't try to rehearse what you will say because I can tell you that it's useless. Just walk up to her and say hello, and then compliment her.
Sometimes putting to much thought into something can only make it harder.. You want to go out with her right? So your best bet is to see if you even have a connection. See if you can talk to her, and if she enjoys talking to you. Most of the time girls don't like just being asked out by random guys (unless they're really hot) So your best bet is to try to make her laugh and just enjoy being around you AT ALL. If you can't accomplish that then you weren't meant for each other anyway.