What do you write in a good secret admirer letter?

I really adore this guy I go to school with, but he's older and I'm not sure if he like me. He a very shy, nerdy boy, but I really like him. However, he's one of those boys who'd never tell a girl he likes her [it would embarr him]. I want to write him a series of secret admirer letters, but I don't know what I should put in them.