Do a basic course and learn as you gain experience, as there is a lot to take in.
Bike shop mechanics are bodgers in my experience, not engineers. The only thing I use them for is wheel building, the last two wheels I've had built, they've (different shops) fecked up a rotor or another part of the hub in the process. I now need to add a wheel building stand to my kit!!
By the time you've done a basic course, ridden your bike through a couple of winters, had various parts fecked up by shop mechanics (spotty kids – normally), and read the odd book or magazine article – you'll be competant at fixing and building your own bike. Plus some expenditure on tools of course!
🙂