How can I be a secret admirer to the guy I like?

Very simple: Admire him in secret. If you start following him around and leaving notes in his locker and stuff, that's borderline stalking. Now, if you want one day to be more than his secret admirer, go ask him out. Just ask him if he wants to get together to do something sometime. But do it when no one else is around so that you get a more honest reaction out of him. Good luck.