Generally you don't. Still it is not unheard of a gal asking a guy out. But traditionally it is the guy who does the asking. Especially at the age I suspect you are. Don't despair though! Even a young gal could say something "simple and innocent" like: "Wanna go to the movies with me?" There is a 1 in 3 chance that the young girl in question likes you a lot. But I think that probably happens to the typical young man at the same rate as lightening strikes. Still, it did happen to me as a 6th grader, unfortunately, I had NO IDEA what I was missing back then. Confidence helps tremendously. Even if you have to fake it. REAL confidence will probably bring the girls to you like sand on sandpaper.
I'm afraid it is impossible to get them to ask you out. Once you build enough confidence, only YOU can make the move. Plus in our world's wacky sense of social acceptability usually the man is the one who ask's out.