No idea where to start

I too know how you feel.. Ya just have to prioritize what you need to do and write it out then stick to your plan.. When I started stripping down the interior of my duster I started with the back and it looked great, then I started taking the carpet out of the front passenger side first. I rolled it towards the drivers side and much to my surprise the floor pan looked in good shape, I then rolled it on over to the drivers side and that's when reality set in.. Rusted completely thru around the seat mounts, pin holed and push your finger the metal in the rear floorboard along with the front where your rest your heels. Since I don't have a compressor to carry air tool loads, I wound up using a "dremel"..LOL... and about 22 cutting discs.. I can say that it gave me a very nice cut line and I didn't even knick or cut any brackets, cables, or brake lines.. Its the old saying "nessecity is the mother of invention"!
PoisonDart gave you some great advice. as well as everyone else does on this forum.
Here's a few pictures, I haven't got it welded in yet but the new is prepped and drilled ready to be welded.. Good Luck and most of all... Have Fun! That's what this hobby is all about...