Every single car owner should know how to do the following:

Change a headlight on YOUR car (certain cars are different, just because you know one, does not mean you know all, especially for the less mechanically inclined). This actually applies to all lights on your car now that i think about it.

Know how to do a serpentine belt on your car. 9 times out of 10, this seems like a more daunting task then it actually is. Not only will this save you hundreds at your mechanics, it will give you a confidence boost next time something breaks.

Understand how to read a check engine code, and do the research PRIOR to going to a garage.

These are just the essentials in my opinion.