Being a writer takes more than a good story and a fancy way of telling it. Very often, a writer can be criticized for their improper use of grammar, misspelling, and using words out of context. Sometimes, a reader will get so annoyed by the writer's inability to use the English language properly that they stop reading! Make an effort not to be that writer. This is especially important for small press and self published authors. You have fewer people looking at your manuscript, so you have to be your own editor.
One of the biggest abuses in writing is improper use of the comma. Here is one of my favorite sites to explain comma use. Purdue OWL