Trying for 12 years to gain weight and no result indicates that the
problem is hereditary. However, prior to putting some weight, its better, you
measure your BMI. It will indicate you whether you are overweight or
underweight.
A strong will power helps either to gain weight or lose weight. Worrying
too much about your weight put a adverse effect on your weight gain plan. So be
relaxed.
Diet: - Select protein and iron rich food. The protein rich food should
include red meat, poultry, milk and diary products, peanuts, soybeans, pulses
etc. You must not be a voracious eater. Gradually increase your food intake.
Between meals, you should take 2-3 snacks. Snacks must be rich with protein. Sesame,
peanut, milkshakes, processed cheese are preferable. If you can, go for protein
powders.
Exercise: - Join in a professionally-run gym. This is a place where you
can increase your weight under supervision.
Hope this will help you friend.