Try out the manual. It will step you through the basics.
You will also want to look at the example files. Assuming that everything is installed correctly, you can get to the examples by going to the Help menu in Pd and selecting examples. A bunch of the patches should start with gem
The best one is gem/01.basic/01.redSquare.pd It puts a red square up on the screen and allows you to rotate it.
gemImage.pd shows how to load in a TIFF file.
gem/03.lighting/04.moveSpheres.pd moves two spheres around the screen. Try the other ones. Most of the GEM objects have test patches which give some information about the various controls for the object.