The Overhand Knot
This is probably a pretty good place to begin to learn tying knots, since it is really just giving a name to something you already know how to do. The overhand knot is really the same as the first part of tying your shoes (assuming you haven't been brought up on Velcro®!). It is very useful to put at the ends of ropes to prevent it from sliding it through holes and the like (hence the classification "stopper knot"). It is also often used within other knots, such as the fisherman's knot.
Knot Video: How to Tie the Overhand Knot
Knot Tying Instructions
Step 1 - Hold the rope out parallel to the ground and cross it over to form a loop.
Step 2 - Wrap the right-hand end through the loop, and pull!