First, be sure you're writing what you know. This is assist you in writing clearly and succinctly about your topic. One of the reasons my first book was about gallbladders is that it was easy to write what I knew so well and lived everyday.
With my other books I had gotten stumped, not because I was writing out of my league, but I had to work harder to get it going.
As an example, let's say you work with computers everyday, but your hobby is reading romance westerns and therefore you want to write in that genre. While you may want to write the fiction books you are finding it slightly more difficult than you thought. Doesn't it make more sense to write about computers?
I'm not saying don't write the fiction book, but if you're desperate to get something out there, then write what you know. You know computers. You know what most peoples challenges are with computers. You also know there could be 100 books out there about computers, but remember, you can know who in those books is or isn't helping people and build off of that with your own material and knowledge.
While you're writing what you know, you can be working on enhancing your skills as a writer. Use your computer book to practice and get better. This will help you when you get back to working exclusively on your fiction book.
What if you don't have a niche or skill you would write about?
We all have a story a tell and there are always people who need to hear your story. Write what has inspired you and keep a running log of those inspirations. Someday you will have enough to make a book and then there you go! You wrote what you know.
If you're not in a niche and just looking to see what's selling...then this part if for you:
On Amazon there is a spot where you can see what the top 100 books are that are selling. This can help you determine what main genre most people are buying.
Look at movie coming out: when the Twilight series came out everything vampire and werewolf was suddenly super popular. If you were a writer who cashed in on that then you would have found that authors couldn't write fast enough for the fans!
Lastly, if you are writing a book on computers and wonder how well similar books are selling, then look them up on amazon. Click on the book and scroll down to where it has the information on the book. It will tell you what number it is in sales for that category.