There are really only five general rules about using commas correctly:
1. Separate members of series by commas.
The colors of the American flag are red, white, and blue.
Standing in long lines, being jostled by noisy crowds, and driving in heavy traffic are three of the reasons why I don't like going to Six Flags or Disney World.
2. If a word group such as a subordinate clause comes at the beginning of a sentence, it is set off by a comma.
When the Vietnam War was over, many Americans were not sure why the war had ever been fought in the first place.
3. Use a comma before coordinating conjunctions like "and" or "but" when the conjunction joins two independent clauses.
I rushed to the drug store last night, but it had already closed.
I stood in line for three hours, and still I never got to buy a ticket for the Spice Girls concert.
4. Parenthetical elements such as appositives, dates, non-restrictive clauses, are set off by a pair of commas unless they come at the beginning or end of a sentence.
He was born on November 10, 1940, in Washington, D.C.
Bill Clinton, who was born in Arkansas, was elected as President of the United States.
Al Gore, a Senator from Tennessee, became Bill Clinton's Vice-President.
5. When in doubt, leave commas out.
Follow rules 1, 2, 3, 4, and then don't add any more commas. Do not try to guess where commas go by reading sentences out loud.