The guys I've come across usually start with eye contact and trying to talk to you more and more. Then they'll tease you and try to make you laugh... but shy guys will usually stick to just the staring. It really depends on the guy... and if you feel like they're flirting just flirt back to let them know that you're interested, too.
If someone is trying is get your attention, they might do silly things AROUND YOU. They will act different when they are around you. They might flirt with you or even mock you. If you notice that they are appearing everywhere you are, they might be trying to get you to notice them. If someone is doing this to you, then you should talk to them more (if you like them, of course). You could also flirt.