Paint something to use as your practice image.   I painted some flowering alfalfa on a black background for this demonstration to show how light affects an image.

To access the Apply Lighting dialog box click on Effects/Surface Control/Apply.   Plain Light (1) is the basic light of Painter™.  To create any other styles of light you must manipulate the controls contained in the Apply Lighting (1a).  These manipulations change the default lights to create more complex lighting schemes.