This basic knot comes in handy outside knitting and crochet, too.
Let the tail of the yarn hang in front of your palm, and loop the yarn loosely around the first two fingers of your left hand.
Pull the yarn attached to the ball underneath the yarn behind your fingers and then through the loop.
Hold the tail of the yarn in your left hand and the newly made loop in your right hand. Pull the tail to tighten, and make a slipknot.