Unfortunately for me, this can mean a lot of work in the editing process. While my stance is that punctuation is the easiest thing to fix in a story, I understand why most editors reject stories that are filled with mistakes. It can take me several hours to edit a two thousand word short story that has the same mistakes throughout the story. (It would be a lot quicker, if I could just hand them their redpenned manuscript.)
Don't use the internet to learn the rules of punctuation; there are a lot of sites that are just plain wrong in what they tell you. Go down to the bookstore and buy a book on punctuation and grammar. Keep it next to your computer while you are editing your story, because I swear to freaking dog that I am going to scream if I see one more comma splice.
If you don't know what a comma splice is, then you need a grammar book. :)