If it is a scratch that Disc Doctor won't do a thing for it- what the disc would need is a resurfacing which could be done at a used game store or some music stores (check your local yellow pages for "disc resurfacing" perhaps). It's more than just a cleaning. This fills in the scratch and makes the disc pretty much like new.
If it is just from dirt/ fingerprints on the disc you can clean it using a soft cloth, wiping from the center of the disc outwards, being sure not to follow the grooves.