I think I fancy a girl ..?

I'm a girl myself and she is one year older than me in high school. I think I have become obsessed with her because I look on her Facebook page every hour and sometimes even perv at her pictures.. And when I see her with her friends I suddenly get jealous and I want to be her best friend only. I feel like crying over it. I once spoke to her on Facebook, but never in person so I don't even know what she sounds like. She's dead pretty and I think I fancy her.. Help.??