It's okay to have more than one crush! But if you're going to pursue a girl, it's probably best to focus on the one you like the best. If you don't know if she likes you, try flirting with her a bit. You might give her compliment or try gently brushing against her in the hall. If she makes a lot of eye contact, she compliments you back, or she goes out of her way to be around you, she probably likes you.