Aerosol adhesive remover
Soap and water
Black shoe polish
Ultrafine sandpaper: 2000 to 3000 grit for wet dry use
There are five layers on any modern car: the clear coat, the actual paint, the primer, and the steel. You need to figure out how far down the scratch goes.
Run a finger across the scratch. If you feel a bump, it's just some material rubbed onto the car. Spray it with the aerosol adhesive remover and it'll probably come off with no problem. If you feel a depression, however, then it's a scratch. Look closely at the scratch. Do you just see a slight amount, or do you see primer or metal? Check the length of the scratch to see if it goes deeper.
Wash the scratched area with the soap and water. Note that in scratches that go down to the steel, you should not try to prime and paint it: you'll just have to wait until the entire panel gets a new paint job. If you see primer in the scratch, fill it in with the paint and allow it to dry. If you don't see primer, skip to Step 4.
Wash the area with the scratch with soap and water to clear away any dust. Then, where you see a depression or bump if you filled in paint, take the sandpaper and carefully sand the area until the depression is even. Run your hand across the panel to make sure the paint is even, and don't hesitate to get in there and sand away small bumps.
Put some clear coat over the repaired area. This will protect it against UV rays and keep it shining.