I really have a crush on some guy but he thinks I am annoying and immature.. What can I do OR SAY to make him think I'm not annoying? I would really like things that I can say to him like "Oh well thats nice" stuff like that, I don't know how though....

You are learning here lifes lesson that not any of us can impress everyone. Hes telling you as nice as he can that hes just not interested at all in you so best to forget him and move on. Maybe on down the road, he may change his mind but for now he wants nothing to do with you so forget him before you really get your feeling hurt so move on.  Good luck

Best thing to do, is that if he thinks you are bugging him.  Cool it for a while.  Say hello, and if you get the feeling he is annoyed by what you have just done or said say sorry to him and say you didnt mean it.  If you want to talk to him, just talk to him like a friend.  Or if he is a sporty person compliment him on his sport preformance etc....Try not to act or make it totally obvious that you have a crush on him either.  Be friendly and not make it obvious that you have a major crush.  We all act a little annoying towards someone we like sometimes though.  If he is still stand-offish to you, cool it down and just ingore him in future.