how do I ask her out???

ok well I really like this girl but I've never had a girlfriend before (im in 10th grade I used to be fat and unattractive but now I lost weight and I've finally started to be able to talk to girls) so I have no idea what I'm doing with this kind of thing I've been talking to her a lot, I've brought her to a movie once am I suppose to ask her to be my girlfriend or do I make a move and hold her hands or what? please help!!!!