“How do I tell a guy that I like that I like him?”
It depends a lot on whether you know this guy at all or not.
If you don’t then it should be relatively simple. You don’t so much say that you like him as just ask him out. All you have to do is strike up a conversation about whatever you can. The conversation can be about pretty much anything, from asking for help with math homework to the weather to a common interest you know that the two of you have. You can even make up a little white lie if you have to (so long as it doesn’t last past your first date). When the conversation lulls, ask him when he’s free. If he responds, then ask him if he’d like to grab a bite or something. Be prepared for him to say no, though, and don’t beat yourself up if he does. In this scenario, there’s no real way for you to lose. The worst thing that could possibly happen is he says “no”, and you’re back to where you started. It’s like gambling in a real casino, but you get to bet with Monopoly money. There’s no reason not to go for the Jackpot, since you can only win if you play.
If you already know this guy, then the situation gets a little bit trickier. If you guys aren’t friends (just lab partners, classmates, what-have-you) then there’s still not much at stake. The only thing you risk is making things awkward during future interaction, but that can be avoided if you can both be adults about it. If the two of you are friendly, then you might be taking a real gamble, since your friendship is at stake. I’d suggest keeping your eyes open for signs of interest on his part, or even ask the opinion of one or two mutual friends. If you’re really bold, you can even spend some one-on-one time to gauge him: try a little flirting, read his body language, watch to see if he gets antsy or he loosens up. If all signs point to yes, then go ahead and level with him. “So look, I hope I’m not making things awkward or anything, but I think you’re really hot/smart/sweet/funny/sexy/etc and I’d really like to go out with you some time.” I’m a firm believer in having as little bullshit involved as possible.
No matter what the scenario, the important thing to remember is not to take any rejection personally. If someone’s not interested, then there’s no point in pursuing anything with them. It takes two to tango, so to speak. So dwelling on any rejection or any relationship that “could have been” is nothing but a waste of your time. You deserve to be with someone who also wants to be with you. You should also know not to second-guess yourself. Sure, you’re allowed to be a bit nervous, but you can’t let it hold you back at all. Confidence is sexy (not arrogance, but own what you’ve got). If he can tell you’re uneasy, it’s going to make him uneasy. If you appear relaxed and confident, then he’s more likely to relax, and thus enjoy your company.