@NWBill Rather than trying to nail everything one photo at a time you need to go out and do the same exercise that all of us have done at one point: try everything.

If you're shooting a picture of the tree use Av mode at f/4, f/5.6/ f/8, f/11.3, f/16 (You can stop there) at ISO 100, 200, 400, 800, 1600, and H. Leave IS on so that the higher f/#, lower ISO shots have a chance at being anything. To keep the shots the same use autofocus (use the center one only to know exactly what you're shooting at) then turn AF off on the lens. You're shooting digital so it's free to blast the entire series.

You'll see how fast you can push the 'film' before noise kills you, the depth-of-field effect of changing apertures and the steadiness (or not) of handholding your lens even with IS. Eventually you'll start shooting moving targets then shutter speed becomes important. For stationary targets the shutter speed is somewhat irrelevant and only allows more or less light in.