Buy a new game. It may be a lost cause. You could try smearing the classic white toothpaste on the disk and try to buff it out with it. Then wash it off. Usually that works for me, but like I said, depending on how many scratches and how deep they are, you might just want to toss it.
Whether or not the game is a complete loss usually depends on the type of scratches you see on the disc. If the disc has scratches that run around the disc in a circle then there was probably too much data loss for it to be any use. I would suggest just buying it used at gamestop or someplace like that.