Commas seem to be horribly misunderstood, overused, underused, and/or outright ignored. This is mostly a tragedy because the comma, when properly used, does wonders!
Commas separate the elements in a series of things (when the series includes three or more items), including the last two. Ex: I bought eggs, milk, bread, and cheese at the store today.
A comma combined with a little conjunction (and, but, or, for, so, yet, nor) will connect two independent clauses (that is a lesson for another day). Ex: I brought a notebook for class, but I left my highlighter at home.
A comma can be used between two adjectives when the conjunction "and" could be used between them. Ex: My cat has a shiny, black coat. You would not use it here: She bought an expensive cotton yarn. (You wouldn't say she bought an expensive and cotton yarn, because you're conveying that that cotton yarn that was purchased was expensive. Make sense?)
These are just a three comma rules. There are more (of course there are) and there are great grammar websites out there that you can reference if you aren't sure. Some of the ones I check (yes, even I double check myself) are:
The Writing Center
Guide to Grammar and Writing
Now, go and use commas properly!