A toilet that runs, rocks, weeps at the base or will not flush.
The parts inside the tank by name, what each failure sounds and looks like, and whether it is a small part or a call.

: it is almost always a flapper or a fill valve, a small part and about half an hour; how to tell which one from the sound and the water line; when it is worth calling. No step-by-step.
What the flapper is, the signs it has failed, sizes (2 in vs 3 in), and the price.
What the fill valve is, the hiss and slow-fill signs, and the price.
Supply valve, fill valve or float: the signs of each and the part price.
A labelled diagram of the tank: each part, what it does, what it costs. Recognition, not repair.
The usual causes (chain, flapper, low water level, clog, vent) named with the honest call for each.
The right plunger (a flange plunger, a cheap part), the signs that a clog is really the main line, and what a plumber charges.
Bubbling points to a vent or sewer-line problem; what to watch for and why this one is a call.
Usually the wax ring (a cheap part); the floor-rot risk; why waiting costs more.
When to replace (crack, age, constant repairs, wobble) and the installed price.