Hello young man! boost your morale first! These days we have obesity as a growing epidemic
, you are lucky to be lean! First of all start on a regular exercise programme to train your body to be fit and use all the essential things you provide in your diet for proper muscle mass development. Take more of protein rich food like lean meats, red meat, egg whites, cottage cheese, pulses, wheat products which help to build up your muscle mass.Do not indulge in taking alcohol, it will invite more problems and you may land in liver
and pancreatic problems.As you adopt these lifestyle changes, you can expect the results slowly over 3 to 6 months from its proper implementation.Above all, go ahead with the will power that you know what you are and that stamina is more important than structure and your inner true self is your guiding spirit!!All the very best!