How to ask a girl on a date?

Ok i met this girl in a shop and i really really like her. She is 3 years younger than me but i dnt fink that will be a prob. I talk to her on facebook and she goes to a different skool. I've talked about all kinds of stuff with her and i really want to tell her that i like her and we need to meet again perhaps in the same shop i met her in. I wont see her in person till that meeting so i have to ask her out on facebook. my question: how do you ask a girl you dnt really know out on facebook. Thanks in advance! :)