First you can find out, if you have the same intrests with her. And then you can talk about your intrests. If you are in the same grade with her, you can suggest her your help with the subjects you are good at. You can also talk with her about you classmates, you common friends and about guys of your school. So many girls like to talk about these topics. Don't be shy when you start a conversation with a girl. Imagine that the girl likes you too and she is glad to speak with you. Try to be polite and friendly , but don't do too many compliments because it can be annoying for girls. Try to seem confident.