How do i gain weight quickly and healthily?

By Healthy Calories: If you want to gain weight, you can do it by consuming more healthy calories such as protein, complex carbohydrates and healthy fish or plant based fat, also weight bearing exercises like weight lifting will help build muscle which will increase your lean or muscle mass. I suggest checking your resting metabolic rate and based on that going on a high calorie zone (40/30/30) diet.