How do you get a car door open when it is jammed shut?

Has the door been hit? If so, a body shop would be the first place to try. If not, roll the window down and spray penetrating oil, like WD40, into the latch area of the door (this is about at the vertical middle of the door, straight down from the outside handle). Let it work in for a day or so, then try to work the handle from both inside and out, and the lock buttons as well. This will free it up in most cases. If not, the latch may be broken.

Sometimes, trying to open a locked cardoor with the wrong tool (such as a Slim Jim) can break the linkage off the handle or the lock cylinder.

If not successful, take the car to a locksmith.