Permanent repair = cut out & replace as already mentioned.
For a less-expensive repair it depends on whether there's perforation. If there isn't, there are products - RustMort and other rust converters or encapsulators - that you can apply to both sides. Dries hard, and after some finishing work you may be able to spot in the color with a rattle-can of the same color code from AutoZone or whatever. Lightly hand-polish the dry/hazy area around the spot-in.
This is if you merely want to stop it getting worse, have metal (not holes), and just need to get some more use out of the vehicle.
You should also look at the bottoms of the doors, rockers, fenders & lids to see if those are corroding.