Best thing to do is let him know you like him. Little hints like a wink or a cute smile. Most guys pick up on that right away and he'll know. Don't do anything big, just little things that he'll notice.
What do I do if I like a guy?
The best thing to do would be to show a bit of interest without overwhelming him. There are loads of ways you can make guy notice you like him.
Rooster already gave you some suggestions, you could smile at him, hold eye contact and you could also try to spend some time closer to him.
If you already know him and talk to him, try to get to know him better by asking some personal questions in order to find about himself and his interests, or something you two might have in common.
Take it easy and don't be afraid to open up a bit but don't be too rushed either: Let things happen and develop naturally. Best of luck! (:
smile at him.