Shirley, you might enjoy this app: http://camerasim.com/apps/camera-simulator/
It will let you adjust all the manual camera settings (ISO, shutter speed, aperture), as well as distance and focal length. You tweak the settings, click the Snap button, and it shows you what your final image would look like, whether it's under/overexposed, grainy, if the movement in the picture was frozen in place or blurred, etc.
It's a good way to understand WHY each setting makes a difference, and how you need to tweak one setting to offset another.