If it's not too serious, sand down to clean metal. Apply rust killer, and then paint. I'd use layers Hammerite or equivalent on the parts that won't be seen and then lot's of zinc primer and layers of car paint on the parts that can be.
Once dry, load the inside of the car door with waxoyl and the outside edge with regular car wax. Build up the coats of wax.
Pay particular care to edges - where the metal is thin and exposed to the elements on more than one side.
Finally, buy cheap mud flaps, cut to fit and attach to the liner on the front wheel arches. This stops salt being thrown onto the sills and doors in future. Mud flaps on the rear wheel arches save the tailgate.
Regards,
Martin
Fix It Again Tony!