The answer to this question is two-fold. Firstly, what's your best answer so far as to what your niche is based on your experience and expertise? Secondly, what is your audience most interested in and responding to? The answer to this will be a sweet spot in the middle. I find that often it simply takes an outside perspective to be able to narrow down a niche. Also remember, you can have more than one niche, as long as you write about them separately. Hope this helps! Happy to answer any further questions.