What do you do when you like a guy that already has a girlfriend?

If he's married or engaged, stay away. If you know for a fact that he's in a long-term, exclusive relationship, you might also do the same. He's already spoken for. Making a move on him would be dishonest. But if he's not committed to anyone, if he's just merely "serially monogomous," then you could make a move on him. Let him know you're available and interested. If he's interested, he'll make a move on you. If you are the aggressive type, you could ask him flat out what his situation is and whether he's dating or seeing other people. His current girlfriend might not give you the same advice <grin>, but hers is not exactly an unbiased opinion. His attitude about dating other women is her problem. All is fair in love and war.