How i can gain my waight as soon as, my age is 26 , hight 5.3", & waight 42?

3,500 kcal = 1 pound: I assume you weigh 42kg, not 42# (pounds). I also assume you're 5'3", not 5.3'. Use body mass index (http://www.Nhlbi.Nih.Gov/guidelines/obesity/bmi/bmicalc.Htm) to calculate normal bmi (16.4 which is underweight). Try to eat 500cal/d more than you expend via exercise. Over a week, that's 3, 500cal or 1#. Eat plenty of fruits, veggies, lean protein, healthy fats/oils. Avoid junk & fast food.