While there are many specific uses for commas, nearly eighty-five percent of the commas used in written English are used in a mere three situations.
If you know the basic rule for these three cases, you can use commas in over four-fifths of the times you need to use commas.
1. Put a comma before a coordinating conjunction that separates two independent clauses.
For more on this, see Commas in Compound Sentences.
2. Put a comma after introductory words, phrases, or clauses in a sentence.
For more on this, see Commas and Introductory Words, Commas After Introductory Phrases, and Commas After Introductory Clauses.
3. Use commas to set off elements that interrupt or add information in a sentence.
For more on this, see Commas with Interrupting Expressions and Commas with Nonrestrictive Modifiers.