How do I become more confident around guys?

Ugh, around any kind of guy I always screw up. A lot of guys have a crush on me at school, so I'm usually able to get a boyfriend with my looks. But when they talk to me, I ALWAYS mess up! That's why I don't get very many boyfriends. I say the wrong things. A guy comes up to me at school and says "hey". We start walking to cl, and I SAY: "Buh-bye" like an idiot. He gives me a weird look, then goes to his cl across the hall. Well, that sucker isn't mine. UGH! Any tips?