U need to take more ptoteinaceous diet....like if u r non.vegetarian,take meat,fish(but not spicy n oily fried),Eggs(white more),if veggie,then consume more Dal items of all kinds,millets,sprouts maximum.&also take Milk n all diary products.But remember all these condumotion should b along with regular exercises only,otherwise, u will become irregularly gatty rather than looking fit.Take sll types of fruits,Nuts.Water minimum of 3-4 litres/day. Inspite of all this intake,stilll u r lean then we have to exclude any Harmonal imbalance r malassimilation.