Congratulations! You just summoned a magic genie, and as a reward, the genie will grant you one wish.
The catch, obviously, is that he's a complete asshole who will try and use your own wish against you. You wish to be the most attractive guy on Earth, he kills everyone less attractive than you. You wish to meet George Clooney, he convinces a serial killer to change his name to George Clooney and sends him to your house. You wish to end world hunger, he eliminates "hunger" meaning "desire for food". Everyone loses the drive to eat, and we all die of starvation. Et cetera, et cetera.
What would you do? Would you wish for something so simple it couldn't have serious consequences, would you wish for the thing you've always wanted, but make the wish so intricate and specific, it would (hopefully) be free of loophole abuse, would you try to use your wish to give the genie a taste of his own medicine, or would you be like these guys [http://www.youtube.com/watch?v=A0y5dHmVRIc] and not care what happens?
You can always take the War Games approach, ("The only winning move is not to play") but the genie will follow you around until you wish for something or die. No one else can hear or see him, but he'll try to be as annoying to you as possible.
Specify what exactly you'd wish for in the comments. If you think your wish is incapable of being twisted, please so say, and I'll try to prove you wrong.
NOTE: No, you can't wish for more wishes, nor can you wish for the ability to grant wishes or other reality-altering powers. Just... no.