Google "from nand to tetris", there are many versions around, text video etc. It starts with a "nand" gate - an electrical device that for all practical purposes has two binary inputs x,y and one output z with this mapping:

  x=0,y=0 => z=1
  x=1,y=0 => z=1
  x=0,y=1 => z=1
  x=1,y=1 => z=0

where usually "0" is represented by zero volts and "1" is represented by e.g. 5 volts.

(As for how a nand gate is actually built -- well, how much electronics/physics do you know? if not much, just assume it magically does. A lego version that works with levers instead of voltages can be found here http://goldfish.ikaruga.co.uk/ornand.html )

(edit: formatting)