Check out some books at the library...or pick up a couple at Barnes and Noble.
Basically, ISO is like film speed...the higher the speed, the less light is necessary, but the picture will be less clear (will be grainy). I shoot at ISO 100 whenever possible...if not, then the absolute lowest ISO I can use in that situation. Less grain = more clear picture.
Shoot in RAW mode....much easier to post edit
Take time to compose your shots.
I have a book called "Complete Guild to Ultimate Digital Photo Quality" by Derek Doeffinger. I would try to find that - it has a lot of great info about getting very high quality pictures with your camera.