use of commas

Reading about punctuation has never been of much use to me. Maybe this isn't the kind of answer you're hoping for, but what has helped me most in terms of punctuation (and other gramar issues) is reading actual books.

And I'll take this opportunity to recommend Douglas Adams' The Hitchhiker's Guide to the Galaxy, as it's not only the funniest book you'll ever read, but it also has some very peculiar grammar at times, which illustrates how to correctly write quirky sentences. For example, here's an extract that demonstrates how an incredibly long sentence doesn't have to sound convoluted: 

The Hitchhiker’s Guide to the Galaxy has a few things to say on the subject of towels. A towel, it says, is about the most massively useful thing an interstellar hitch hiker can have. Partly it has great practical value — you can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble‐sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft down the slow heavy river Moth; wet it for use in hand‐to‐hand‐combat; wrap it round your head to ward off noxious fumes or to avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mindbogglingly stupid animal, it assumes that if you can't see it, it can't see you — daft as a bush, but very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough.

As was previously said, explaining commas is a complicated task but here is another simple way to remember the difference between the two.

A semi-colon is use to join two sentences that could otherwise stand on their own. For example: I got up this morning; I was already late for work.

A comme can NEVER be used to join two complete sentences (that would be a comma splice). Instead it is used to either separazte elements of a sentence or to separate items in a list. For example:

After I woke up this morning, I realized I was late. ("After I woke up" is a prepositional phrase and could not stand on its own as a sentence.)

The car was painted blue, white, and gold.

This is a VERY simplistic explanation but I hope it helped somewhat.....