If you are writing with the goal of publishing your work, you need to know your target audience - specifically!
Many authors fail to take this into consideration when they are writing. They write what impassions THEM, but they never stop to think if there's a large audience out there who also wants to read their work.
Make sure what you are writing is actually SELLING. Go to your local bookstore and ask what genres are selling best. See what's currently on the shelf.
Profile your target audience - what is their age, gender, income level, education level? Will your work appeal to a specific region of the country or world? Where does your target audience like to shop, where do they go for fun, and most important, what sites do they frequent online?
Bottom line, if there is not a NEED for what you are writing, it will never sell.
And if you do not know how to reach your audience, you will not sell books, either.
So do your research!!!