u are under weight
u must find out the reason behind it
it may be heredity ,nutritional ,psychological ,pathological ,thyroid
or other hormones related .
u should check urself
as far as diet is concern u need to take high energy diet
ur total calorie intake must be higher then output
u need to eat more dryfruits ,milk and its items ,fruits and non veg if u can .
eggs and milk are essential for u
there are medicine but to be taken after dr consultation.