Hi. You are suffering from under nutrition. Your ideal weight must be according to the formula. Calculate height in cm, deduct 100 from it and you will get your answer. So ideally a person with 160 cm of height must weight around 60 kg +/- 3 kg The beat way to increase your weight is by a good diet. A diet with 60% proteins, 30% carbohydrates and rest fats. So I suggest you increase your protein intake and do regular exercise.