Memorize the mnemonic device FANBOYS, which represents the following coordinating conjunctions: for, and, nor, but, or, yet and so. These are the most common conjunctions in the English language. Conjunctions connect different clauses or major parts of sentences that have more than one subject and verb. When these conjunctions connect two parts of a sentence that have their own subject and verb (independent clauses), you need a comma before the conjunction.
For example: Shelly doesn't like crowds, so let's go home before the show starts.
If the conjunction does not combine two independent clauses, you do not need a comma before it.
For example: Shelly doesn't like crowds or music.
Place commas between each item in a list of three or more. This separates the items and makes it clear that they are part of a list. For example: "I ate potatoes, ice cream, and bratwurst." You also use commas to separate multiples adjectives that describe a noun. For example: "She was a quiet, mysterious horse."
Use commas to block off interrupting phrases in sentences. These phrases, sometimes called nonrestrictive appositives, make a comment about a major part of the sentence and add detail, but are not necessary to understand the sentence as a whole. For example: Robert, who smelled like peppermints, walked through the library. Surrounding the phrase with commas signals the reader that it is not essential and shows what the words are referring to.
Place commas after introductory words and phrases of sentences. Introductory words like "next," "then," "of course," "second" and "eventually" require commas after them to signal their relationship to the sentence. Introductory phrases like "In the beginning" also require a comma after them. These phrases generally start with a preposition or worlds like "although" and "if."
You should leave out commas unless you can justify their placement. Never place a comma in a sentence simply because the sentence seems too long. Frequently this is a place for end punctuation or a conjunction.
Consult a grammar and punctuation handbook for specific doubts, instances and more examples of proper comma use. Read newspapers and novels to see how published writers use commas, and see if you can find mistakes.