How do you let your crush not think you're annoying?

Don't crowd them. Don't hang around them uneccessarily, to stare at them from across the room. Many young people are embarrassed when someone crushes on them and makes it excessively obvious. Be casually friendly to then, but only in the way that you are friendly to other people. I know it's hard, but try to keep your "crush" on this person to yourself.