How can i let the guy im startin know that i like him.....?

ok ive been going to math camp n theres this guy who i just started talkin too n i kinda have started to like him n it seems like he likes me to we flirt n stuff haha but slight problem he has a girlfriend but still i still he likes me or somethin so how can i let him no i like him n how can i no if he likes meee im going to try n get his number bfor camps over next week so yea n thanks