Hrm...we girls generally are pretty open to being asked out...and although we are scarier than anything you could ever face across the 40K board, we're generally not too desperately evil at heart.
Go for something that seems like not a big deal, like, "I've really been wanting to see *insert name of movie here* and I was wondering if you wanted to go and see it with me"
Or if you're talking 'ask-out-as-in-be-commited-to-one-person'...then my confusing response is this...usually, if you think you're in the position to do this, the girl thought of it before you and has been dying for you to ask...so you're pretty much home free.
Just be cool, and be yourself. The worst thing that happens is that she says no, in which case you gained experience...good luck!!