First follow rj713's advice, you ought to check different types of both CDs and DVDs, different brands, and r vs rw, if none work, try an optical driving cleaning disk, you can get them most places, but you can also get them [http://www.amazon.com/Game-2-DISC-Laserblast-Optical-Cleaner/dp/B00099EBZW|online]. You can also try cleaning by blasting compressed air in there with a can.
If all these fail, get a new SuperDrive [http://www.ifixit.com/PowerBook-Parts/G4-Aluminum-17-Inch-4x-SuperDrive/IF155-042|here].