There are two reasons for writing: for love or for money. If you're writing the book of your heart, and you don't care if anyone buys it, go ahead and write your dream. On the other hand, if you're one of the millions of authors who want to make a living with what they write, you're going to have to produce what the readers want to read. You've got a better chance at making a living income if you write to a larger audience than a small one. 
Genre fiction is a broad area, with hundreds of sub-genres, each with its own fan base. The key to a successful career in genre fiction is to find a niche that you can write in for years, and one that has a sustainable audience with a Goldilocks size: not too big, and not too small. Narrowing down your genre takes time, but the results are worth the effort you'll put into research.
The Bottom Line
A career in genre fiction can last for years or decades, so you'd better be sure you pick something you like to write. If you can't stand military science fiction, it doesn't matter if the audience is starving for tons of new books. You'll hate every day you spend writing about it, and that's not a sustainable plan. Your first step in choosing your genre is to find three or four types of book that you like to read. Don't automatically throw out unfamiliar niches. If you've never read a cozy mystery or a western romance, give them a try. You already know what you like to read when you want to relax. See if you can find another genre or two to that list. Limit your list to four main genres, at the most, and make sure they're all niches you enjoy reading. Don't worry about the plots for now, just see if you enjoy the way stories unfold in each genre.
Narrowing it Down
Finding the right niche is a pure numbers game. Amazon is the biggest bookseller online, so their numbers are the best ones to exploit. Start doing research by checking the Amazon bestseller lists. Search for the top 100 ebooks on their site. Once you find that page, you'll see genres in a list on the left. Click on the main topics and you'll find sub-topics under each one. Drill down the topics to find one you'd like to write, then look at the top 100 books in that niche.
Ignore any books written by bestselling blockbuster authors. Using only the data from independent authors, look at the top three books to find out their sales rank on Amazon. Are they ranked between 500-1,000 on the paid lists? Good, they sell enough to be marketable. Next, avoiding the blockbusters, check number 20. If this book runs just under 10,000, you've got a winner. Enough fans to sell a comfortable amount of books, but not so many that you can't get your toe in the market.
If the numbers don't work, move on to another sub-genre until you find one that does. This is the sweet spot, the underserved fans who will buy your books and wait impatiently for the next one. Stick with this niche for the best chance at creating a solid, long-term career in genre fiction.
Keep the Faith and May the Force be with You!
