I agree with the first poster. It's a good idea to figure out who you like before getting any more feelings wrapped up in it. I wouldn't worry about anyone liking anyone else until you know if that's what you're up against. If you do like a guy who is in a relationship or likes someone else you should probably take a step back. Here are some things you SHOULDN'T DO if you find yourself in that situation

on't try to get in the middle of his other relationship, or between him and the girl he likes, and don't talk negatively about this girl or start rumors- these things are completely childish.
DO: Be his friends. If he needs to talk, let him talk and listen to him. If he wants to hangout, let him hangout with you- but be very honest. If he is in a relationship let him know that you have no intention of coming between him and his girlfriend and you pose no threat to them. Also (this may be hard) but you could reassure his girlfriend also that you aren't going to cause problems and you are simply friends with this guy. The last thing you want is for his girlfriend to go up against you because she is suspicious.
All in all, the most you can be with someone who already has a girlfriend is friends. If you're flirting, or going beyond a friendship- it's cheating, and you don't want to start any drama.