I really like this guy but he's going out with my best friend... But I really like him what should I do?

Your going to have to let their relationship play out as if you do steal him not only will your best friend probably hate you but you started your relationship on a bad precedent that might come back to haunt you as he might be bitter by the fact you pulled him away from a relationship that could have been good for him.

Get to know him maybe he isn't the guy you think he is, and if he is you just have to wait.

But I really like him what should I do?
You carry yourself with dignity, and respect, and don't cross the lines of decency, that's what you do. Then you can enjoy his company as a friend, with no regrets, nor retributions. See how easy that was.

Jokingly remind your girlfriend that when she's done with him, you're going to take "your turn". Later then it really happens, remind her you told her honestly ahead of time.

As for the guy, you probably need to keep your mouth shut until they're done. That's just the way it is when you have real friends... and good character yourself.