This question is killing absolutely every blogger wanna-be, this is often the biggest excuse of not starting a blog - not having a clear idea on what topics to write about.
We all have hobbies, we all like something and spend a lot of time studying it, it might be football, cooking, fishing, designing, art, music, whatever. People are passionate about various things in life, and it is awesome. You definitely have interests that drive you crazy when someone starts to talk about that. You have already thought about it in the Categories phase that I've mentioned in this post earlier.
Here are a few actionable steps:
1. Ask yourself what you care about the most in life? Write it down.
2. Why are you caring about it?
3. How people will benefit from your writings?
4. What value you will bring to your readers?
Don't write about the things you don't understand. Write about the things you have been geeking out a lot. Choose the topic that you can call yourself an expert.
Another option is to be a curator. You can learn on the way and write about what you have learnt, provide readers with resources, discussing mistakes you did, revealing potential shortcuts etc.