a smash repairer or panel and paint guy

it all depends on how much rust and what you want your car for.

if you want it to last a long time and you want a nice finish then it is a bit of work but if you are fixing it up to sell get a wire wheel put it on a drill

brush all the rust area until you get it back to metal
buy some rust killer
fill with some household expanding foam ( for fixing cracks in walls and spaces)
then rub it backfill over with bog
spray with body filler
spray with primer and then spray with the paint of your choice


but if it's to keep

go out and buy a bodywork book
and give it a shot.