well, you could be straight forward and tell her you like her. Or you could do it old fashioned and send her a note. Or you could wait until you're both alone and ask her out. While doing this, you should also find out something she wants and give it to her as a present. Complementing her also helps build a relationship.
I'd start by talking to her and saying hi everyday and just making basic conversation. The way she reacts should let you know if she is interested. Once you are more friendly you can just casually say hey I was wondering if you'd like to go hang out sometime and then read from there and you should know =)
i have this problem too but im a girl and my crush i a guy haha. but yeah back to you just ask her out girls wait for the day a guy asks them out dont keep her waiting its not good to keep girls wait just sommen all your courage and walk up to her and talk then ask her out