What do you do if you like a gril but you already have a girlfriend?

You have a commitment to your girlfriend that you need to put first. The other girl is pretty crappy to tell you that she wants to go out with you knowing you have a girlfriend. She apparently has no respect for boundaries, which should really bother you if you are considering a relationship with her.

It's too late, you are going to end up being a dick to someone in this scenario. It also sounds like, either way, you are going to end up regretting whatever choice you make. Talk to your current girlfriend. I know most people out there have problems with a completely honest relationship, but you might want to see if she is willing to open up the relationship so that both of you can see if you two are right for each other or if you would both be better off with someone else.

You're going to need to dump your girlfriend, to go out with the other girl. You can't have them both.