How do I get the back door on the driver's side open?

This can be incredibly difficult and will very likely cause further damage to the door jam paint than the belt buckle has done already.

I would go inside the car, grab a hold of the belt and pull hard while pushing or kicking on the door to wedge the belt out. DO NOT KICK OR PUSH ON THE WINDOW. Have someone on the outside of the car hold the door handle in the open position while standing back. Then forcefully push and/or kick the door while pulling on the seat belt to make progress as much as you can until the door finally opens.

You may end up needing a new safety belt afterwards.