Always place a comma before the conjunction that precedes the final item in a list.
Do you prefer chocolate, vanilla, or strawberry ice cream?
Janice taught Tom how to sew on a button, change a tire, and make a martini.
(Editors call that final comma the serial comma, the Harvard comma, or the Oxford comma.)
NOTE:
Newspapers and magazines typically omit the serial comma, allegedly to save space. But dropping that final comma can occasionally confuse or mislead readers:
Ralph spent last Saturday night chasing girls, drinking beer, discussing tattoos and fighting.
Without the serial comma, readers can’t tell if Ralph merely talked about fighting or actually threw some punches himself.