Always use a comma when you can justify its use by one of the reasons below.
Commas follow all introductory elements in a sentence.
Commas separate items in a series, and there will always be one less comma than item. A series is made up of three or more elements, and it is possible to have a series of words, a series of phrases, or a series of clauses. Commas set off all parenthetical elements. Two commas set off parenthetical elements in the middle of a sentence and one at the end. Parenthetical elements are those words which are not necessary to complete the meaning of the sentence.