Here's my best very basic advice about commas. Put them where you'd pause for breath or effect. There's a world of difference between:
"Shoot John!" and
In one sentence John gets shot, in the other he is instructed to shoot. That can make a big difference to your reader, as well as to John.
If you are unsure about your commas, try making a PDF copy and running it through Adobe Acrobat in "read aloud" format. (Acrobat 7 and above will read it aloud to you.) Then you can hear the places where commas come in. Pick up on unnecessary pauses caused by too many.