Try flirting with your object of desire and get their attention. Let them know that you are interested in them. Don't do anything forward like tell them straight up "I like you" just yet. That doesn't go very well, trust me since I've done it.
When you like someone, you also try to hang out with them as long as you can and as much as you can somehow. If you're shy, then you probably just stare at them a whole lot. You don't do this on purpose most of the time. It kind of just happens.
But this all depends on the person. Some people have a different approach to this. Each person has their own style.