Start poking at it with a blunt tool like a phillips screw driver. That will give you an idea of how bad it is. I'm thinking you are about to do some welding.
^^^Above is so true!
I use a big right angle grinder with a 4 inch wire CUP to also determine what metal I have. And like the other guy said, thin metal is hard to mig weld, and rusty metal won't weld!
And also like said, there are quick "fixes" and real fixes! I always prefer to leave as much original metal as possible.
There are ways to actually make repairs to these reinforcing ribs that can last many years, with a car in a garage, but lipstick on a pig is what it will be called, so I won't go there.