contrary to popular opinion in this thread DONT ask her to a movie. Go somewhere where'll you'll have more of a chance to chat to eachother and get to know eachother. Its important to try to have a decent conversation on your first date, because you'll know if you h ave things in common, if you can make her laugh/smile. Or if its becomes just too much effort to think of things you two can actually talk about, its a sign that you dont have much in common or aren't feeling open with each other, so it probably wont work in the long term.
As for the asking her out, be confident strike up a conversation with her, if its busy, ask her if she wants to go for a walk n talk, then when you feel ready ask, or if you dont feel ready, man up and force yourself. Jus say something like "so do you fancy goin to *blah* at the weekend, just us two?"
If she says yes, act cool, dont get all excited n shit
If she says no, try not to seem to bothered, also if you get an excuse try to think it through later, how did she seem when she said no. If she sounded a little bit upset when saying it, it means she probably did have a real excuse. If she gets flustered then she is probably thinking of something to not hurt your feelings, so let it go if its that. Or if its just an "I like you, but not in that way" try to move on.
Good luck
Bennis out!