DO SOMETHING THAT YOU LOVE!
A niche is a subject that you may know a lot about or have a passion for.
Having a niche that you are passionate about makes it easier for you to write your articles, given that you will be very knowledgeable about the subject and it will also attract other people to your website who have similar interests. They would be able to contribute comments and ideas to your blog and potentially, if you become an affiliate for products which flow out from your niche, they could generate commissions which you would earn by selling things through your affiliate links.
Whatever your subject of interest is, be it fishing, cooking, golf, music, education, Forex trading, computer technology or absolutely anything at all, you can be sure that there are thousands of people out there who would enjoy visiting your website.