My main advice to you (which should cover the majority of cases), would be to simply learn how to reformat and reinstall Windows. Beyond that, it would be more of knowing how to handle specific problems, both software and hardware related. For the software side of things, knowing how to keep your computer malware/virus free, defrag your HDD, keep your HDD and OS clean and tidy, etc. are huge.
The majority of computer systems never run into hardware issues. If you're somebody that knows how to maintain a system (examples above) and reinstall Windows, that will cover you 99.9% of the time. As an added bonus, if you knew how to install memory and video cards, that would cover the majority of times people need to touch their hardware. Building a computer is ridiculously easy and easy to learn, but if everyone did the above things there would be very small demand for computer "repair."