You need to get a "door poper." There are little white tabs that hold the door panels to the door. The poper slides under the panel and you pop off the tabs like using a lever. Go to any car stereo install shop and they will have one, or you can buy one from Snap-On or other tool suppliers.
You can pop the doors by hand, but the door popper is the safer approach, without breaking the retainers. You'll also need to take off the molding near the window sill and will have to pull off the clip on the window crank (if they are manual windows). You can do this with two screwdrivers (carefully, without stabbing your self [img]/wwwthreads_images/icons/smile.gif[/img]) or you can use a rag and "work" the clip loose.