You really jumped into the middle of shit storm with this project. Taking something like this on with no experience is a brave thing. I will try to give some choices for you here. The first question I have about this project is this. Is the rust just in the sheet metal or has it gone into the sub structure. If it has you may just want to get rid of the car because it can be very unsafe. That out of the way, if you are just doing sheet metal here are some suggestions.
1: You could do the welding thing, but as you said you don't have a welder. You could rent one but if it is just a clunker I wouldn't worry about it.
2: You could do the fiberglass thing but it can get expensive.
3: If you can get an old hood or something to cut pieces of sheet metal out of. Then cut to size, usually you want to make sure it is into an unrusted section of the metal. Cut out as much of the rust as possible. Then pop rivet the new piece over it and learn to sculpt body filler.
4:This is considered the cheapest and easiest as far as I am concerned. Tape a piece of cardboard on the outside of what you want to fix. Shape the cardboard the best you can. Then get a can of the spray foam insulation and fill the hole up. Obviously this will not work if it is in an area that things have to work by. Such as the doors where the glass moves. If you can get to it you can put a piece on the inside of the quarter panels to keep the amount of insulation to a minimum. You have to use your best judgement at this process on where you can use it. After it dries just peal off the cardboard, smooth out, and put a  little filler over it. If you have any questions just ask.