I am 19 years old now and i am very thin and had very less muscle.what i should do to increase my muscle and to test?

Nutrition & exercise: Good nutrition & regular exercise are key to health. In order to gain muscle, do opposite of those attempting to lose weight. In other words, consume at least 500cal/d more than expended. Still need to exercise so that extra calories don't become fat. Consider consuming eating more protein w/each meal, up to 1g protein per pound of body weight. Don't worry about testosterone.