Answer:
Remove the interior door panel. You will see a metal rod connecting your interior and exterior door handles. Most of the time the rod comes loose from one or the other. Reconnect the rod. There's not much space so you might need a pair of needlenose pliers. If that doesn't work, there is a trip lever toward the rear of the door. The lever can be stubborn but it will open the door. Any vehicle service center should be able to open it for you.
Access Through Opened Window (easier if it will work)
If the door is jammed shut, it will not be possible to remove the door trim on most cars without destroying it. However, if you are able to roll the window down you should be able to insert a thin piece of metal (e.g. wire coat hanger, "slimjim") and manipulate the door handle rod inside to pop it open. This is possible only if the reason behind the jam is mechanical and not structural.