How can I stop being shy?

I'm really shy, but I need to break out of it because i'm going to high school next year, but its REALLY HARD FOR ME! When a person walks up to me, and tries to talk to me, I turn stiff, and weird. When im suppose to smile, I like grimace, and even though the person is talking, I dont know what to talk about. I'm afraid that I will be a loner, because im not really good in conversations. What should I doo?