What to do if I like a guy who is in a on again off again relationship?

I have liked a guy named Derek for a couple of months now. He is dating a REALLY bratty girl named Kaitlyn. I think about him alot, but am afraid to talk to him. What do I do. I think he likes me. He stares at me alot, and his friend an I talk alot. Every time we are talking , he will come over and join our conversation. What does this mean?