Talk to her about casual topics like school or shows on tv, but don't be SO casual it's boring. Crack a few jokes to show that you're fun to be around and ask what she likes to do. You're conversations might not be too long right away, especially if she's shy, but once you start talking to her more often (almost everyday), she'll start some conversations and realize how great you are to be around. Ask her what she does after school and maybe ask her friends what she does,
BUT be aware that her friends will probably tell her that you talked to them and asked about you, so maybe you might not want to, unless you think she likes you too and you want her to know that you're intrested in her. If she doesn't know you, she won't be able to tell that you're nervous unless you're shaking all over and are studdering, so just try to picture her like a friend you're talking to instead, but make her seem more important.
Help her out if she's having problems with something and she'll thank you and say you're a great guy, ect. Whatever you do, don't act too nervous and ask about too personal of topics such as where she lives or stuff like that. Ask that later on when your conversations are longer. Also, make eye contact with her and if she breaks eye contact with you right away, look away. That means she's nervous but she still likes you. If she holds eye contact for a while, she's confident in talking to you.

If you're having a rough day or are not yourself (mad, sad, ect.) don't show that around her. Act happy around her and pretend to be in a good mood because after all, no one likes someone who is mad around them and not having fun.
Basically, everything bloomia said is right on and that's how you should approach the situation. Good luck!
