I wrote some reflections in https://jamesg.blog/2023/11/27/technical-writing/ that cover the "what", "why", and a little bit of the "how" of technical writing.

If you have already decided "yes, I want to become a technical writer!", write, a lot. If your job doesn't involve writing, start a blog. Write about the technical challenges that interest you; the web, databases -- anything! Having public examples to which you can point will look great to employers.