Human growth hormone is made naturally in the pituitary gland of humans, the gland is deep inside the brain just behind the eyes.
How To stimulate Human Growth hormone
Human growth hormone is secreted in short pulses during the first hours of sleep and after exercise. To ensure maximum stimulation, get quality sleep and engage in some form of physical exercise.
Although Human growth hormone is secreted naturally throughout life, some people suffer from human growth hormone deficiency. In this case they might need to inject HGH. It has been used to help stunted children in UK grow normaly since 1959.
Sources of the article, simple : BBC human growth hormone medical notes
Another good source, detailed : Wikipedia growth hormone
Plus : wiki how to grow tall