Major rust under door. Ideas on fixing?

[godzillarb]

5 minutes later...



I noticed some serious rust under the passenger side quarter panel this morning, poked it with my finger, and everything fell apart. Ugh. Started cutting and it just gets worse. Rust keeps going past where I cut in the picture, but is getting less and less torward the front. The metal support with the holes in it is rusted through and basically fucked. The part that really worries me is the backside of the sheet metal is rusted as well. Seems like it'd be a lot harder to fix / get to?

I'll get more pictures later, but any ideas on tackling this? I'm decent with a mig welder, just not really sure where to start.