This text covers only my experience, or things I believe to be true. It would be best for anybody that is planning to do something towards repairing anything to read as many sources as practical. If you start seeing the same things over many sources, then that thing might just be true. In this day of world-wide sources of information, you truly need to take any source with a grain of salt until you can determines if it rings true. Once in a while there will be a source that rings true all the way except for one point. Sometimes people get a different idea of some things – and reading lots of sources will let you know what "most people" say. Sometimes that person behind the source source either knows something that others don't, or thinks something or some way that others don't. It is up to you to check things out. Nobody knows everything.
In order to start, you need to know something about electronics. How much you need to know can be debated, but you should at least know what electrons are and how they act. That is called, I believe, "The Electron Theory," which is covered pretty well on WikiPedia:
(more about the history at: www.philsoc.org/1962Spring/1526transcript
A necessary understanding you should start out with is the concept that electrons flow through conductors and not through non-conductors.
To understand tubes and semiconductors (transistors, diodes, and FET's), you HAVE to understand that electrons flow from negative to positive, and are particles.
I remember a particular saying to help you understand heat and energy in electronic devices. It was "Voltage is how badly electrons want to get some where, and Current is how many of them are going."
You can know that is true by trying to pass a large number of electrons (current) through a wire at certain voltage and see how hot things get. That is a type of "friction" that happens that is directly proportional to the formula Current x Voltage = Watts. Heat is a factor of Watts.
Amps should be studied carefully with constant knowledge of the dangers of high voltages and high currents. Current flowing through the human body is not a pleasant experience, and can wound or kill you. Current flowing through things makes them hot, which can burn you. And high-voltage sparks are both shocking and burning, so you have to be very careful.