Calculate your weight as 1 Kg on every inch of your height. Whatever weight you are short of you need to gain it on right places like arms, chest , shoulders. thighs and calves. I am not talking about being a body builder but yes a good body has a healthy mind.
I went thru this trauma of being underweight for 10 years and then I joined a gym with good instructor. it definitely paid me .
I would suggest, if you are able to put some money into it, join a gym with good instructor. do a survey across your city.
If you want to do it at home then jog, skip and do push ups daily.
Add up a daily diet of
2 glasses of full creme milk , sugarless, not warm but on room temperature with two spoon full of honey.
2 bananas, one each after meal.
If possible take 4 egg whites.
4 Brown bread with butter and sprinkle sugar on it.
2 table spoon full of desi ghee , in dal (must in one meal), (not with honey)
1-2 Boil potatoes
Divide this diet across your regular meals.You can not bulk up w/o increasing your diet.
Do not take medicines to bulk up. i would suggest 1 LONGIFENE tablet 1/2 hour before meal.It helps in stimulating your hunger with no side effects.
if you are consistent in your efforts for 6 months you will get what you wish for.