The thing NOT to do is go on the internets and ask advice...
but oh well, I will contribute to this with what I have learned from 26 years of being retarded over girls.
first off, just do what feels natural. If it feels like you should ask her out, do it. If it doesn't, don't. Women, people in general actually, tend to be more open to an idea if the person behind it is confident and sure, so if you are going to be all squirmy when you ask her, it may have bad results.
Test the waters a bit, if it seems like you have a shot, go for it.
Also, you may look about 10 times cooler if you just say something like "I cant stop thinking about you" instead of asking "will you be my girlfriend?"