A warmup is designed to stretch your body out and prepare your body for running, increasing your heart rate. A cool down is designed to do the opposite, decrease your heart rate from being as fast as it is now to a moderate rate.
When you run and you're almost finished, slowly decrease your speed and breathe deeper. Then, when you decrease your speed to a good walk, simply walk. Walking is a very good cool down. Also, focus on maintaining good breathing control while you walk.