Depending on the scratch. Very fine scratches can be buffed out with regular white toothpaste.
If, like Vivo says, you can feel them with your fingernails, the scratch needs to be filled in. When my car was "keyed" few years ago I bought small can of paint in the right colour, filled up the scratch until it was higher than the original paint, then I scraped it even with a razor blade and finally buffed it out.
If you have deep scratches in unpainted plastic/steel, a sandpaper is your friend. Start with around 400 and end with 1000. For very deep scratches or cracks in plastic fill them up with material similar to this two part repair material.
http://www.wurth.co.uk/product/replastkit (This is professional product and devilishly expensive. Very likely not worth buying to repair just one body panel.)