Although I don't have a personal blog, I used to write a lot on SitePoint. Here are a few things that can help you get started :
Write about things you are excited about. For example, when I started writing Angular apps I was very excited about $apply() and $digest() - These are very basic, but many developers don't know about them and their internal details. So, you will help a lot of developers if you can write about stuff you care about in a lucid way.
Beginners and intermediates mostly look for tutorials/articles that explain how to build real world stuff. If you are learning a new technology and trying something new you will probably build some simple apps (e.g. movie rating, todo app etc). Why not write about them? You can upload your code on GitHub and let your readers follow along easily.
If you are evaluating multiple competing technologies, you may write about that as well. There can be multiple tools to achieve the same task and it'll be great to see a well written article that discusses the key findings of the author, pros/cons of each tool etc.
Write about some weird bug that you encountered and explain how you fixed it.
Be humorous and include images (funny gifs if possible). People don't like to read dry and dull textual content.