According to your height you should have 68 kg.Take proper nutrition through balanced diet.Eat extra amount of food frequently.Use bigger plate.Take whole cream milk.Use weight gainer supplement.Add ghee,cream,butter,cheese,honey.jam,jelly with foods.Eat cake,pastries,chocolate.Take dense fruits,vegetables.Use extra refined fods with unrefined foods.Eat lots of protein to gain weight.Use more healthy fat in cookinh.Take extra nuts,seeds.Avoid smoking,alcohol.avoid fast foods as far as possible.Drink lots of water but not just before or during meals.Do regular exercise like push ups,weight lifting and other muscle building exercise which will help you to gain weight as well as to build muscle.