hey! Alright first off: just start by smiling at him in the hallway or in class. Don't stare, but smile genuinely. Next, start waving to him in the hallways. Not crazy-lady-big-overhead-wave, but just like a hi-how-are-you kind of wave. Next, ask him a question in class like "do you know what the hw is?" or something along those lines. Then just keep talkin to him in class, if you're ever in a group project, take FULL advantage of that. Smile a lot, prettily, not psycho-ish. Tease him slightly, when you're more comfortable talking to him. Find an excuse to touch him, ask him playfully for a piggy back ride if you're walking, or find any excuse to lightly touch his arm or hug him or something.
Guys are attracted to confidence and they hate insecurity. Sad, but true for us girls. They want you to flirt with them but they don't like skanky. Wear girly clothes, well-fitting tight jeans and a fitted top or a cute denim skirt & flats, you get it. A lot of guys like girls in hoodies, but make sure it's a fitted hoodie. Straighten your hair even if it's already straight, or curl it. Do something with it. Wear a little more makeup than normal (unless you wear a fair amount already) with special focus on lip gloss and mascara and blush. but keep it natural and do not go overboard. when in doubt, go for clear gloss, dark brown/black mascara and light pink blush. However, if he's emo/scene/skater/preppy/gangster/etc, dress a little like his style. But girl-it-up a bit more. Find excuses to hang out w/ his friends. If you want to start out as just friends, be up for anything. Don't gossip, discuss drama or 'girl' things, and don't be squeamish. Be flirty, laid-back and laugh at what ever joke he attempts to make. Even if it's disgusting. Or stupid. Just go with the flow.
It's okay if it takes you a little while. I was the shyest girl EVER two years ago and I gradually changed. I'm really flirtatious now and very outgoing. Just don't become someone you don't want to be. Don't force anything. Be you, but the best you.