a good buffer, with a fine grit sandpaper to take the finish all the way to bare metal. then use body filler and a rubber scraper to fill in the low spots. use a fine sandpaper to smooth the filler down to match the metal. dust off the car completely. then use a spray gun to shoot on a thin coat of primer.
There's actually a few ways of doing this, depending on what you're after. If the car's paint is not peeling, you don't need to strip it down to bare metal. You can wetsand the car, with 400 grit wet or dry sandpaper, clean thoroughly, and apply the primer. Many places like to use a primer-surfacer, which acts as a body filler for minor imperfections. Then the primer-surfacer is wetsanded, and the car is cleaned again. After that, it's a good idea to apply a sealing primer over the primer-surfacer.
A word of warning though, primer does not repel water, and many times will absorb moisture, leading to surface rust if the car's left in primer long enough.
To get a more definitive answer, you'll need to tell us what your goal is in priming the car, what you're looking for in terms of finished product quality, etc.