If your goal is publishing something, I think it's great to keep an audience in mind. But if you're struggling to get a writing habit going, forget about all that stuff and just write. Thinking about all that stuff is why it's so hard to get started! It's the root of writer's block. I talk about that a bunch in the guide.

That’s usually the same advice I give to people when they ask the question in the context of personal blogs: just write. About whatever. Anything you find interesting to you is a worthy subject.