Stop being negative?

I'm a very sarcastic and cynical person. I see the negative in almost every situation. I've been told I'm very callous. I dont ever expect to be a perky happy person, but I'm tired of being how I am. I think my outlook and the things I joke/laugh about push people away from me. I guess I'm a mean person. How can I be positive? I dont think I'm depressed, but I wouldn't say I'm happy.