How to Fix This Door Panel

It seems like my driver door either had a dent pulled or just a real good hit to the door panel, but in either case the result is this:
What would be a good way to fix this so my door doesn't rust from the inside out? Rain has to be getting in because lower most crack creates a lip that water can run into. I have no body work experience; the only thoughts I had would be to try to get body filler in behind the crack and pray, or replace the door panel or the whole door and hope to match the color.
Any tips?
What would be a good way to fix this so my door doesn't rust from the inside out? Rain has to be getting in because lower most crack creates a lip that water can run into. I have no body work experience; the only thoughts I had would be to try to get body filler in behind the crack and pray, or replace the door panel or the whole door and hope to match the color.
Any tips?