Fixing nicks and scratches as soon as they occur not only makes your car look better, but prevents any place for rust to begin attacking the metal.
First determine your car's paint code. It's found on a badge under the hood, in a door jamb, inside the trunk or in some other hidden location. If you can't find it, check your Owner's Manual, a service manual, or call your car dealer.
At the auto parts store, you match your paint code with the code in the paint-matching application guide. Often the same paint color is used on different models, even different brands. For small repairs, purchase a small touch-up bottle with a brush built into the cap. If the scratch goes down to bare metal, you should also purchase primer and a metal conditioner.
Wash the repair area with mild soap and water, rinse thoroughly and then dry with a lint-free cloth. Remove wax, grease and oil with a wax remover, acetone (fingernail polish remover!), or lacquer thinner. If there is any rust, scrape away as much as possible with a single razor-blade, then treat with a rust remover followed by a rust converter. See HANDLING RUST
When you are down to bare metal that's rust free, apply a metal conditioner according to container directions. Then brush on two or three coats of primer, waiting five minutes between coats. Allow the primer dry for thirty minutes before applying any color coats.
After thoroughly mixing (shaking) the color touch-up paint, apply two or three coats in one direction, with sufficient drying time between coats. If the original paint has a clear coat, you should apply two very thin coats of clear, to achieve proper color match.
Let the repair dry for at least a week and then use fine polishing compound on a very wet rag to smooth out the repair. Be careful not to rub through the color. Finally, follow up by waxing and polishing to achive the final, protected finish.