how do i fix this?

Your options are-
1. Hammer and dolly and then mig weld the missing areas, backing the hole with copper.
2. Cut out the area and weld in a piece of a donor car.
3. Find a repair plate from you local auto parts and weld it in.
4. Weld it up and drill/ file/ dremel your own hole.

http://www.rustrepair.com/KEYHOLE-REPAIR-PLATE-DOOR-M1375.HTM