How do I ask A guy Out that knows I exist, but never talks to me?

I really like this guy, but he's popular. I'm not. He's funny and cute and nice, but we never talk. What do I do? I've never asked anybody out before. This would be my first time. What if he says no? It will be all over school, and i'll be mortified! i've never even had a serious, or any, boyfriend. But I really want one. What do I do? Help!