Once you've identified the type of scratch, move on to the repairs.
Surface nicks. Spot-clean and dry the area. Add a light, even coating of car polish to a clean piece of terry cloth (or use the product's applicator). Apply it by pressing gently while buffing with small, circular movements. Let the polish sit about 5 or 10 minutes until it's dry--read the package instructions for the exact time. Remove the residue by rubbing it off with a clean terry cloth and using the same circular movements. The polish will fill the nick and, once buffed, blend in with the surrounding area. Note: Check your owner's manual to see if your car has a clear coat. If it does, use a polish made specifically for this type of coat.
Chipped paint. If you can't see any signs of corrosion or metal in the scratch, then you can just cover it with touch-up paint (found at automobile supply stores and car dealerships). Start by washing and drying the area. If the car has been waxed, use a wax remover when you clean. Prep the scratch by sanding it and any loose paint along its edges with mid-grain sandpaper (120-grit or higher). Finally, give the area another cleaning to remove any sanding dust. If the scratch is on a vertical surface, attach newspaper all around it with masking tape to catch possible paint drips.
To apply the paint, pour or spray a small amount into a small container (like a jar lid). Some paints come with applicators, but you can use whatever you're comfortable with, as long as it's small enough to be precise (small watercolor brushes, the paper ends of cardboard matches, and toothpicks all work well). You'll need to apply it in two or three layers, building it up higher than the surrounding paint. Allow half an hour or more for the paint to dry between applications.
After you've waited 24 hours for your layered application to dry, sand down the bump of paint to match the surrounding surface level. This should be done with superfine sandpaper--1,000- to 3,000-grit (you can find these in auto supply stores, usually in sanding block form)--and the work can take a while, at least half an hour. Sand slowly and gently, inspecting the area regularly until it's even with the surrounding surface. Once it is, apply polish and wax (if needed), and you're good to go.
Metal scratches. If the scratch goes down to the metal, with no signs of corrosion, prep the area (clean, de-wax, sand, clean, and dry again), then apply a coat of primer paint, available at auto supply stores. Buy primer that has a rust-inhibitor, just in case. When it's dry, apply the colored paint, polish, and wax, as described previously.