Do NOT use a regular paper towel. And if you clean a DVD or a CD, only wipe from center to outside, NOT in a circular pattern.
Many stores have DVD/CD cleaning fluid and cloths (the same work for both).
If there are actually scratches (rather than smudges), you can repair superficial ones with a CD/DVD repair kit. If, however, the scratch is on the label side, and you can see through the DVD/CD when you hold it up to the light, it can't be repaired.