What’s the ONE piece of PC-building advice you wish every beginner knew?

Since a lot are mentioned, this one is very helpful. Most mice manufacturers use something called OMRON switches and it's usually what is first to fail on them. You will recognize this as what's called a double click issue: when you try to hold the button down and select multiple items in a folder it will almost immediately disengage. With nothing more than a tweezers, some rubbing alcohol, and a lot of patience you can usually fix them. These switches are basically a small copper contact that is tensioned by a loop in a groove. When you click the mouse, the copper makes contact and maintains clicked until it is disengaged. Unfortunately, copper contacts plus arcing energy typically means corrosion - especially in humid environments. In other cases, the tensioned bit will often just slip out. Usually the screws are under the sticky feet which should come right off. Pry open the two small black boxes, but be careful as there's a very tiny white piece of plastic that you cannot lose. Under that you should see the little copper arm. There's a bit of a trick to fixing them that feels like one of those metal logic puzzles. There's little 0.0393701 inch grooves that the copper arm just sets into - there's like 3 of them that you have to get simultaneously on a 1/4 inch by 0.0069 fathom pad. It's pretty tricky to get the hang of and it will feel futile, until all of a sudden you just figure it out. Then usually the second goes very quick. Because of that, I highly recommend multiple mice and just setting one aside until they all break. I personally have three that I switch between and spend less than an hour fixing them every 5-10 years.