In addition to FrameMaker, Word, Flare, and so on...
Look at job ads and see what companies are asking for.
You might see a lot of companies are asking for Markdown + GitHub. I recommend this Udemy.com course to get started with that: Git and GitHub for Writers. (I also recommend all the courses that Peter Gruenbaum teaches on Udemy.)
Consider learning DITA. You can start with Learning DITA. The book DITA Best Practices will help you, too. oXygen is a popular DITA editor and they sell a personal license (or you might qualify for an Academic license if you're in school).
Slight aside, think about buying (and reading) Global English Style Guide. What you learn from that book applies to writing technical content even when it's not translated.