I am not a professional educator, but in general I think it is worth to start with basic computer literacy: identifying parts of a PC, being able to explain their overall functions, difference between hardware and software, and what kinds of software a computer can run (firmwares, operating systems, user utilities etc.). This would also be a perfect time to develop practical skills, e.g. (assuming you are a normatively-abled person) learning to touch-type and perform basic electronics maintenance, like opening your machine up to clean it and replace old thermal compounds.

After that taking something like "Operating systems fundamentals" on Coursera would be a great way to go on.

It really depends on your goals, resources and personal traits, as well as how much time and energy you can spare, and how do you like to learn. You can sacrifice and old machine, boot Ubuntu and break it a bunch of times. You can learn how to use virtualization and try a new thing every evening. You can get into ricing and redesign your entire OS GUI to your liking. You can get a single-board computer like RaspberryPi and try out home automation.