How do you know if the person you like likes you back?
Maybe if the boy/girl keeps on looking at you. Or maybe the boy/girl gives you stuff or gives you attention. Or the boy/girl talks to you a lot.
If you like her back, talk to her (which may be hard), flirt with her, and try and get to know her. If you can't do this, all girls have a less popular, usually less attractive friend who you could get to know and then get to know the person who likes you. If you don't like her back, then try not to be too harsh, and tell her that you don't like her.