OK what do you do if you really like this girl but she is dating someone and that guy was your friend but now you don't talk to him? And what if the girl you like went over at your friends house with you and you and the girl held hands but she was dating some one else?

I am guessing there is one girl and one friend
So I am assuming that the girl likes your friend but you and her went to his house and she was holding your hand?

I would say back off until she decides what she really wants.

Boys fiddle with other people's girlfriend, men respectfully deal above the table by letting their intentions be known but by not turning the desire of his heart (the girl) into a cheating tramp.

Tell her you're interested in her but a gentleman does not accept "back seat" to another man. Tell her that as soon as she's ready for a mature relationship with someone who's nuts about her (that's you), then you're ready, too. Tell her to call you when she's completely, absolutely available.

Tell her not to wait too long to decide since you can't guarantee the universe won't intervene and put another awesome girl in your life... if you're still available when that happens, then you'll of course be interested in the available girl, especially an awesome one.

Got to spread jbeaucaire... gotta spread it like butter.

Props to jbeaucaire.

Don't sneak behind your friend (even if you guys don't talk) and mess around. That makes you "that guy"... and nobody likes "that guy"... trust me.

Step up. Tell her you're interested, but you know she's with another guy.

However, realize that if you do this, you will most likely lose that "friend"