No idea where to start

Don't forget to check for the source of water entering the car that caused the rust in the first place. A body shop guy on LI once told me that a daily driver up north will likely have rust on the driver side floor pan because of snow tracked onto the carpet all winter long. It will less likely have rust on the passenger side from that source unless there is always someone riding in the passenger seat all winter.

Notorious for water entry are leaking firewall gaskets and leaking heater box fittings.