I started out with minimal mechanical knowledge, but a strong interest and passion in cars in general. I remember my first oil change on a 1986 Pontiac Grand Am with the 2.5L Iron Duke when I was about 16, I drained the oil then proceeded to start the car and run garden hose water through the engine for about a minute so as to flush all the old oil out. Seemed to make sense at the time, I remember questioning myself but thought "what harm could possibly come of it?" I filled the car back up with oil started it and although it had a ticking noise that wasn't present beforehand, it otherwise ran just fine.
Fast forward to the present day and I do *ALL* wrenching on my vehicles, except those services I don't have equipment for-- like tire balancing, alignments, etc. Actually in the last 15 years or so, I'm pretty sure the only thing I've had a car in the shop for is tire service, alignments or A/C related. And now I have the tools to do the A/C myself. The reason I do it myself is primarily these reasons:
a) I live very frugally and resent paying someone to do work I can do myself
b) interest in cars, and I actually enjoy it though it can be incredibly frustrating at times
c) I have had bad experiences with mechanics misdiagnosing things and generally being shady or doing unsatisfactory work
Having an interest in something and learning it go hand and hand, without one or the other I believe your doomed for failure or gonna have a miserable time. Two things were key to learning, one is having time (a spare vehicle, so you don't have to rush to get a car back on the road) the other is space (a garage, carport, etc.)
There were lots of jobs I've done that were learn-as-you-go and forums helped immensely. I prefer forums wholeheartedly over Youtube, and actually can't stand watching a You-Tube how-to, because there's often too much fluff in the video and you can't really have a back and forth with questions as you might in a forum. Sadly it seems good help in forums is getting rarer by the day and more self help is only available on Youtube. Lots of time spent in junkyards has been incredibly helpful-- seeing how different manufacturers do things and you can often see how things work just by looking at partially disassembled cars. The junkyard is my happy place, if I go to grab a simple part it's almost guaranteed I'll spend a couple hours there checking stuff out. Good therapy. Most of my mechanic work / learning when I was young (say 18-25) was done via Chiltons or Haynes manuals but I've found they aren't too helpful as most try to cover too wide a variety of cars and options-- for example my Chiltons book for my '85 F250 covered model years 1980-1996, and the zillion different engine options available in those years, so you might have a single paragraph relating to your particular engine, or the illustrations show a 4.9L I-6 instead of the 7.5L V8 you're working on.
I don't have a great deal of money invested in tools, the most common tools I have came from Harbor Freight and I've been pleased with them. As long as you use the right tool for the job, the consumer grade tools seem to hold up fine. There have been plenty of occasions where I've had to pause a job to get a certain tool, sometimes expensive ones, and I simply suck it up as an investment that will make this and the subsequent jobs easier.
My dad didn't have an interest in cars, so I never had a mentor or someone to watch. But after 20 years of doing things myself and admittedly making a few mistakes along the way (minor things thankfully), I'm confident I can tackle most any job including engine / transmission replacements.
