"From my big beautiful warlock brain, welcome to 'Sheen's Korner' ... You're either in my corner, or you're with the trolls"

This quote by Charlie Sheen is a humorous way of saying that he is the leader of his own corner of the world. He is inviting people to join him in his corner, and cautioning them that if they don't, they will be with the giants. He is indicating that his corner is a safe haven from the trolls, which it is a location of charm and intelligence. He is likewise recommending that he is the leader of this corner, which he has a huge, beautiful brain that is capable of making sensible choices. He is basically stating that if you wish to become part of something great, you ought to join him in his corner. He is likewise warning people that if they do not join him, they will be with the trolls, which indicates that the giants are not a desirable group to be connected with.