@Gail: in terms of using lighting for ambience and terrain, try adding a couple of layers above all the others, named Highlights and Shadows.

Flood fill the Highlights layer with white (or a very bright color, like a pale yellow that's almost white). Set the layer mode to Overlay. Add a layer mask, and flood fill the layer mask with black to conceal everything in the layer. Switch your colors back to black and white, set foreground to white, and then use a large-ish very soft brush to brush in highlights on the objects below. Adjust the opacity of the layer to taste.

Flood fill the Shadows layer with black (or a very dark color, like a dark blue that's almost black0. Set the layer mode to Multiply. Add a layer mask, and flood fill the layer mask with black to conceal everything in the layer. Switch your colors back to black and white, set foreground to white, and then use a large-ish very soft brush to brush in shadows on the objects below. Adjust the opacity of the layer to taste.

If you need to intensify a highlight even further, duplicate the Highlights layer, wipe out all of the changes on the layer mask of the top Highlights layer, and then brush in the higher highlights in the appropriate spots. Ditto for deepening shadows.

If you make a mistake, you can always undo the changes by switching back to the opposite color (using the keyboard shortcut X) and wiping away your changes. Or use Undo/Redo, of course.

If you need precise, sharp lines in your shadows -- like, say, light shining through an open door -- you can always select the shape you want using the polygonal lasso tool and then flood fill that section of the mask with white (to reveal that area) or black (to conceal it).

I'm not entirely sure if that's what you were asking for, but I hope it helps you.