If there are scratches, many electronics stores have a 'CD/DVD repair kit'. It works by sanding or polishing the DVD with a very fine grit material. It will actually remove some of the plastic on the DVD to return it to a readable state. It will not work on deep scratches. Once you have polished the DVD, put a protective cover on it so that if it gets scratched again, the cover can simply and easily be replaced.
You can also use toothpaste or Brasso.