I write about any topics that have always interested me and they start to fall into groups after you've written a few, which is nice because then you can link one to the other.
I also keep a notebook and just jot down ideas as I have them. And then when I have time to write I pick one that interests me at that moment. Sometimes ideas that I thought were good when I wrote them in my notebook seem sort of stupid a few days later. I would really suggest doing this too.
I think in time your niche or a few niches will pick you. Just start writing.