If you're already friends with them, just try to get closer.

If not, which I can't really understand why you would have a crush on someone you don't know, here's what you do.

Try not to seem too obsessed with them, but at the same time, don't ignore them. Greet them once in a while, and try to make them start conversations with you, so it doesn't seem like you're obsessing over her. Basically, just eaves drop on them while they're talking, and find out what they like. Then, without her being aware that you know she's listening, some time later, say something about it with your friends and they might try to join in. Example, you find out they LOVE froggies, but they don't know you know that. So one day, while talking with some friends and your crush being near, discuss froggies with your friend. Your crush will be all, like, "ZOMG ZOMG ZOMG I LOVE FROGGIES" and you can start talking with the and try to show interest.