I understand how you feel.
It would have been helpful if you would have mentioned your weight and height.
Being lean is a gift.
People who are obese are struggling to become like you.
Gaining weight totally depends upon factors like diet, workout
But if the condition is familial then it would be a bot difficult to gain weight.
Diet: that you need to take to gain weight
Start your day with having 2-3 eggs followed by a glass of milk with a spoon full of sugar or honey followed by 2 bananas, then have breakfast consisting of cereals, for lunch you may have your food of choice but not junk food, in the evening take snack that are adequate in good fats or you may have egg with milk then 2 bananas, for dinner have light meals.
Workout: hit the gym and ask your trainer to train you with workouts that help in building muscle mass.