It's a very difficult issue, and one that people actually argue about.
A comma used to separate the last two items on a list is called "the Oxford comma", although it has nothing to do with the English city of Oxford. Some people say the Oxford comma is always required, other people say it's only required to avoid ambiguity.
And that's the problem: there is no definitive answer. If you get a job in publishing or journalism, you will probably have to follow a "style guide" which tells you what rules to follow for spelling and punctuation, including the use of the Oxford comma. Each publisher has its own style guide, and that, I'm afraid, is the best answer anyone can give you.