Use them when you are listing things and need to break up the items of the list. Also use them when you are making a small pause, but don't use them for small and long breaks near each other -- use full stops for long breaks and use semi-colons or dashes for breaks that should be longer than commas but shorter than full stops. By breaks I mean breaks in thought, and/or in speech if you were speaking the words. How about try writing something a bit long then we'll give you some feedback and try to help you understand the underlying concepts that way.