This is an abstract from my book How to Start a Blog
Picking a niche is how you can define what your little corner of the internet will be all about.
It can be all about you, your hobbies, passions, and stories. But then not many people will find it very useful. Maybe one post can solve their problems, but will they ever come back? Probably not.
Because your blog’s not about you, it’s about your readers.
Pick a niche. A small subject.
Say you’re a programmer. You can write a blog about the C++ programming language, write 2 posts a week, and never run out of topics for 20 years. It’s probably even a niche that’s too broad, not much of a niche.
A better niche would be C++ programming for embedded devices. Or using C++ to build games.
Then filter out this group of people a little bit more. Using C++ to build a 2D game engine.
You can write about that for years, too, but this time if a person is interested in this subject, they will bookmark the site and download everything they can because you are the expert (or at least more of an expert than they are) and they can get a lot of value from you.