About what should I write on a blog

First, when you decide to make your niche blog, must understand that not all niches are searched on the net. Or that some niches are already over full.
The difference between a personal blog and a general one or niche is huge. But chances to transforming your blog into a BRAND is higher to a blog of niche.

What niche should I choose?

Well I think you should choose a niche on your specialization or one of your hobby. (If you are specialized in catering - write about it. If you specialize in social asistence - write about it. If you are specialized in the construction of houses - write about it. If you like flower growing like hobby - writing about it. And so on.)
In this way you can make your hobby a profession for you or your specialization a brand to others.
However, to be a brand are required and some other things.