How can i gain a weight iam 19 years old but my weight is 39 kg so what can i do?

Calories + protein: The addition of calorie dense foods such as nuts, whole grain pastas, raisins, whole milk, pure fruit juices, butter, avocado, and complex carbohydrates is a healthy way to add more calories to your diet. Lean proteins are also important. Exercising to build muscle mass will increase weight and metabolism, and improve overall mood and health. Good luck!