The best way to GET confident ... Is to ACT confident.
So I like a guy and he is very shy and he doesn't really talk with his friends and I have a crush on him, the problem is I'm to shy myself to approach him and just say "hello", is there anything I can do too boost my confidence to talk to him?
Just make a secure plan of action first. Plan out what you're going to say multiple times, practice in the mirror, and tell yourself that it's easier than you think. Practice your facial expressions. Look approachable, sound confident, hold your head higher than normal. Don't fidget, keep your muscles relaxed. This helps me a lot, since I used to be shy around guys, too.
When you see him at school or something, compliment something about him. You could start with a joke, too. Or maybe even a question about yesterday's homework. These are simple ways to build a friendship. It's working for me since I already made a friend using this tactic.
Good luck, and be brave!