Hmm.. Well try to comment on her clothes, for example, maybe say this: "Wow that color(What color she's wearing) on you looks really good." That worked on me but i don't know if some girls like that. Don't try to stare at her, she'll probably think your creepy.
That's great that you can talk to girls. Talk about your classes, hobbies and all fun stuff to talk about. If you really like her, ask to go to the movies, double date or just alone, go hang out with friends together or anything you want. Just don't go overboard on the first time.
If you did a good job the first time, you can ask her on a date. (Make sure you have permission though because you can get into lots of trouble) If your old enough to go to a nice place to eat you can go on a nice date. If your in 6th-8th grade, just go to a nice movie alone and have a good time. If your under 6th grade, please don't do anything like that. You can just hang out in school together.
Just be nice, friendly, helpful, sweet and whatever you think. Don't be shy to ask her out.