The foundation of all knitting is the slip knot. Once you master that, the rest is child's play - well... just about :)
Since every cast-on method starts with a slip knot, let's get that out of the way first.
Hold the short end of the yarn in one hand, the long end in the other. Make a loop and bring the long end of the yarn around to the back. Pull the yarn through the loop creating a new loop, and tighten the knot.