Learning Mechanics

[Razgruff]

I trained on trucks that went into the detail of engine gearbox axle rebuilds and recons.
You can't beat the practical experience of stripping and rebuilding stuff, but you need to have the theoretical background knowledge to understand what you are looking at and what it does.

I would start by learning systems and components, Carbs are a good place to start, lots of theory and a small component that can be stripped on the coffee table 

There are loads of good sites on the net for learning the theory of various parts and I'm sure you can pick up many bits for buttons at auto jumbles and such, for just taking apart and seeing how it works.
I wouldn't go for a whole non running bike or basket case until you feel confident that you know how each bit works and it's job within the whole system. else it could get expensive as you lose focus on why you brought it and get the "I got to get it running at all cost mentality"

Don't skip electrics, majority of faults will somehow be electrical related, learn how to use a multimeter, it will become a good friend