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It all really depends on the spot of rust you're dealing with, each situation may be a little different in how you have to tackle it.
For nearly all of my repairs on this car I've used my ancient Miller Sidekick MIG with Lincoln L56 .030 wire and 75/25 gas. It was made long before digital readouts so I can't say for sure on power settings, you'll have to feel it out. For the thin sheet metal .025 wire may be a better choice but I've been welding for quite a few years so I manage.
Exactly like brndck said, if the rust hole is huge or in a complex formed area you'll probably want to cut out a good patch panel from a donor car. I wire wheel the rusty area with an angle grinder to find out where it really stops and cut out a nice big square area with good accessibility a few inches past that, in a spot that will be easy to weld and grind smooth later. Lay it over the rusty spot, trace, and use a cutoff wheel to remove the bad panel. If you're MIG welding them together you should leave a tiny gap between the panels, don't make it fit too tight. A few magnets or panel clamps hold everything until you tack it in.
I almost always butt weld the patch panels together, and grind both sides just about smooth (no thinner than base metal) with a 120 grit flap disk after welding. You may need to work the metal afterwards with an appropriate hammer/dolly to stress relieve so you don't get oil canning on flatter panels. Spray self etching primer and smooth with body filler afterwards. Don't forget to zinc spray / weld-thru primer for panels that will be inaccessible after welding, and coat with cavity wax after paint.
If the rust hole is small or in an easy to form area I just make patch panels from the same thickness mild steel sheet. I use poster board or paper to make a template that fits back to good metal and then use good hand snips to cut the steel, then some bending in the vise or a little hammer/dolly work to shape the patch so it matches perfectly.
For light surface rust with no holes or flaking, Naval Jelly usually does the trick.
Also consider buying a center punch and spot weld cutter bit set if you're getting into any panels that can be swapped out whole or some that need to be removed first to get to the real rusty panels underneath. The kind that look like miniature hole saws work best.
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