A car door that is difficult to close or which won't close at all could be the result of a sagging door or a problem with the door latch. Door latches can be fixed in minutes with a few tools, but a sagging door may make professional repair a requirement if you are inexperienced. Check the latch first to save yourself time and money and recruit a friend for a second opinion and assistance while you work.
Look for a rotating latch with prongs on the edge of the car door. The latch should be rotating downward, not upward. Lift the door handle upward and check to see if the door latch is still in the "closed" position. If it is, have a friend hold the door handle upward while you use a screwdriver to manually push the latch so that it rotates downward.
Close the door; if it will not stay shut for a short period of time, have a friend hold the door closed inside the car. Stand 10 feet back and examine the door's alignment with the car body. If the door does not match the rest of the car's alignment, put a 2 by 2 wood block below the door's lower hinge.
Push on the door from the outside to bend the lower hinge, examining the door's alignment after every push. When alignment is restored, stop pushing. If alignment is mostly restored, adjust the fitting between the striker and latch.
Use a wrench to loosen the striker on the door frame, but loosen it enough only to move it. Push the striker around until the door latches properly when closed. Do not allow the retaining nut on the striker to fall when loosening it.
Loosen hinge mounting bolts if the sag is severe and the previous steps do not fix the door's problems. Support the door's bottom on a crate, using a screwdriver or awl to mark the proper alignment around the hinges. Loosen the bolts a little at a time, making one adjustment at a time. Check your work after each adjustment.
Adjusting the striker could result in more problems if you allow the retaining nut to fall off and disappear, so loosen bolts slowly and carefully. If Steps 1 through 3 do not solve your problem and you do not have car repair experience, take the car to a professional for repairs. You may end up doing more damage to the vehicle otherwise.