As a member of academia, I don't think I'm supposed to do this, but I've used Wikipedia on plenty of occasions to brush up on my electrical basics.
You will want to look at basic DC circuits first:
Components:
Resistors
Capacitors
Inductors
Concepts:
DC Voltage
Current
Resistance
Equations:
Ohm's Law
Kirchoff's Current Law (KCL)
Kirchoff's Voltage Law (KVL)
Then move on to the basic digital stuff:
Semiconductors
Diodes
Transistors
It will be a lot of reading - and you definitely can't learn it all at once - but this should get you started.
I'd recommend letting AC systems wait until college since (1) there are none in your Vex controller
(unless you are using a power adapter) and (2) you need to have a strong grasp of complex numbers, phasors, and frequency-domain transforms to be able to do the fun stuff.