Auto body repair & detailing: door closing, latch, back door

Question
I recently bought a 97 cavalier for $400 because it was hit on the passenger side both doors were damaged I replaced both doors the front door is fine but the back door does not align up right their is a gap between the door and the body at least a half inch. Also you have to slam it really hard to close or it won't. How can I fix this problem?  

Answer
HARD TO SAY WITHOUT SEEING THE DAMAGES....IT COULD BE THAT THE POST IS
IN...HOW IS THE GAP AT THE TOP OF THE DOOR ?. YOU MAY HAVE A BAD LATCH.

I WOULD TAKE IT BY A BODY SHOP AND HAVE THEM LOOK AT IT TO TELL YOU.
THEY WOULD PROBABLY ASK FOR A FEW HOURS OF LABOR TO INVESTIGATE.