I'd second what others have said and go with a micro like an avr or a pic. Tons of open source support and a small system you can totally "own" will help you un

I'd second what others have said and go with a micro like an avr or a pic. Tons of open source support and a small system you can totally "own" will help you understand not just the code but how computers execute code at the lowest human-legible level.