I started out helping my dad change the oil, spark plugs and all that easy stuff on his 67 BelAir when I was little, Thats all my dad knew how to do with cars, the rest I learned watching my great uncles work on the farm trucks and tearing things apart and remembering how they worked before I tore them apart, Anything that was electrical or had a motor lasted me only about a week before I tore it down, Needless to say they rarely worked after that, My first real project was when I was 16 and bought a car that was to be used for a dirt track racer, I was taking an auto mech class at that time learning how to rebuild trannies. Bought it from a guy in the class and started stripping it down and prepping the body for the body shop, That car made me learn real fast about prepping bare metal, doing minor fiberglass repairs, all stuff I learned from my uncle spending the summers in his body shop watching him. That camaro also was my learning block for everything else, our family mechanic had just retired around that time so my dad asked him to come over to take a look at the car and what I was doing, he spent the next week over there with me showing me how to set the timing, rebuild a carb, and many other small repairs. A few years later after I got out of the military the car lost all power, was backfiring under load, couldn't get it above 15mph going to work one day, well stupid me thought the engine with its 186k miles on it had finally given out, so I went out bought a crate 327 to replace the 350 and started the swap not knowing what I was doing, only thing I had to go on was a chiltons manual and a champion video on pulling an engine, well a week later the engine was swapped and the car was back on the street. The family mechanic came over and looked at the old engine and told me only thing wrong with it was the timing chain jumped.
I guess what I am trying to say is my steps on getting the knowledge to do all this is just a love for working on cars and finding people that are willing to show me little things as I go, Also there are a lot of great books out there that will help you out and teach you a few things, but the best teacher I had was just going out there and doing something on the car and learing from my mistakes, I knew what I screwed up a mechanic could fix, Sometimes its a costly lesson but now I rarely have to take a car to a mechanic unless its fuel injected.
Trial and error are the best teachers I have found out. If you can't afford to pay a mechanic to fix your mistakes start on something smaller like an old lawn mower engine or an old motorcycle, Engines are basically the same concept, start from the bottom and work your way up to the more complex engine motor.