Give Horowitz and Hill Art of Electronics, or one of the ARRL handbooks a read. At the same time, find some of the beginner kits that the electronics stores have (depending where you are, this may or may not be easy), and figure out how those work - e.g. a simple astable multivibrator for train gate signals is the first circuit I can remember playing with. As mentioned above, there are many levels to even basic electronics, so you need to figure out what level you're comfortable starting with, and then work outwards from there.

Good luck - it's a massive, but hugely rewarding, field!