How do I flirt?

Touch him. Not lewdly, of course (unless I suppose, you want to be lewd). But put your hand on his arm, or wherever is appropriate, when laughing at his jokes. (you do laugh at his jokes, right? and listen to him when he talks about whatever it is he likes to talk about? and ask him questions?)

Find silly ways to put your arm around him. Find playful ways to push him, or mess up his hair, or to lean on him when you're sitting side by side at lunch or wherever. It's a little juvenile, but hey, you're under 18, and presumably so is he. Which makes you both technically, still juvenile. So run with it.

The more you touch him, the more he'll think about you touching him. And about touching you. Which is what you want, right? Combine it with the communication and listening and laughing and smiling and stuff, and guys eat that stuff up!

Wink at him and blow a kiss without the blowing part.

First you should have built up a normal friendship with him so he'll know something is different. Second, it's not really in what you say, it's body language. Smile at him, look at his lips, play with your hair, touch your neck when he talks.