A few pictures would help greatly for giving you sound advise.
To start with, you will need to get rid of the majority of the rust by sanding. 80-180 grit on a DA will get it knocked down pretty quick. You're not trying to remove the rust entirely at this point, but rather making the surface somewhat smooth. Sanding will not get all the rust out of the pitted areas, so don't worry about it at this stage.
Once you have the surface smooth again, you can start cleaning is thoroughly with a good rust remover. There are perhaps hundreds of companies out there selling rust removal products, and each one will tell you theirs is best. In my experience, they're all about the same. DuPont 5717S is an old standby as is Rust-Mort. If you look at the 'ingredients', you'll find that they all use phosphoric acid. This is what you want for removing the rust.
This is painstaking process and will take several hours, if not days to get to good shiny metal, but you CAN get there. It just takes tons on HARD WORK!!!
Unlike what others have advised, POR-15 and it's counterparts are all gimmicky crap! as is most of the JUNK sold by the Eastwood Co.
Your best bet is to have it dipped at Redi-Strip, which uses a non-acid based formula for removing paint, rust and anything else but good ol' metal from your cars body. For about $1200.00, you can have nice clean metal to work with. You'll know exactly where the problem spots are and with the aid of a few pics, we can help you get started replacing bad metal. With just a little practice, you can metalfinish your cars body and not have to worry about that nasty body filler stuff. When you get to that point, I'd be more than happy to talk you through the process.