Ok, first things first.
1. Open the door , grab it by the bottom and lift up on it. It shouldn't move. If it does, the hinges and pins are worn.
2. If hinges are ok, close the door and check the body lines where the panel gaps are. They should line up. If not, you will need to adjust the door. This is done by loosening the bolts holding the hinges onto the door frame and the door itself, BUT, DON'T DO THIS YET!!!
3. If the alignment is off, sight down the door and see if it is standing out from the fender and/ or quarter panel. This will determine how you proceed with the adjustments.
4. If you can provide some pics looking straight at the door when its closed and pics looking down the side from the fender back and from the quarter panel forward , we may be able to help some more.