Kitten: to quote the doors of the Heart of gold "Glad to be of service" (if you think im talking rubbish please read hitchhikers guide to the galaxy)
Ted: Kitten is right though, your approach should vary depending on how well you know someone. if anything asking out someone you dont know is easier, because there is not as much discomfort if you dont talk to the them again after asking them out. whereas with a longer running friend you dont want to mess up what you've already got (hence why i irritated the hell out of my gf's best mate for a month or two before asking my gf out).
good way would be to invite her to watch a film or something (this is presuming you know her at least to talk to) and see how she reacts when it is just you two together no-one else to feel embarassed in front of. (if ur lucky they give you hints that they like you, such as nearly fall asleep with their head on your shoulder, though i think generally most girls arent that obvious) just make sure its a romantic film either at the cinema or just rented out on dvd (not quite so romantic but hey) if she says yes to watching the film just the two of you its a good start, just see how things progress. also just ring her up to chat to (make sure she knows you've got her phone number otherwise she'll get scared, oops
), i was ringing my gf a couple of nights a week for a chat before we were going out, means you get to know them before jumping in at the deep end.
i have now run out of useless things to say so will end my ramble here.