Okay, I bought a 1993 F150 ext cab and I need to fix some rust spots some places, and I am low on money, so I am pretty sure I am going to go with bondo to repair it. I have looked up some tutorials, but what I don't get is will it just blend in with the color of the truck or do you re-paint it or how does that work? Here is a picture of the truck:

 

bondo is a putrid tanish color. You'll need to paint over it.
Also, if the rust is real bad, cut it out. If it's not too bad, sand it off.
There's also this stuff you can get that turns rust into primer.
The way that works, you get all the loose rust chips and as much of it off, sand the place down a bit and then hit with this liquid. They have an aerosol can version too, but it's no where near as good. I suggest doing this in the holes you're patching to prevent more rust from forming around the bondo. Bondo won't rust, but if the metal it's bonded to rusts out, the whole glob will fall off.

If you're trying to increase resale value, one thing people do to help cheat people (NOT saying you should do /that/, but..) If you have any substansial amount of metal dust, mix that in with the bondo. it will help make the bondo be magnetic, and enough of it will help stretch your supply of bondo.

That looks to be one of Fords standard non-standard colors... so DupliColor may have that color in spray can. It makes going over the bondo easy. It might not match 100% since the paint is 15 years old on your truck, but it'll still lookbetter than bondo tan :S