Girls who want to gain weight quickly and healthily should concentrate on eating a diet high in calories and nutrients. Include high-calorie foods in your meals and snacks, such as nuts, seeds, avocados, and nut butters. To promote muscle building, include foods high in protein, such as lean meats, fish, eggs, dairy, and legumes. For long-lasting energy, eat complex carbs like oats, sweet potatoes, and whole grains. Include good fats as well, such as those found in fatty fish and olive oil. Eat larger portions and snack frequently throughout the day to optimize weight gain. To efficiently grow muscle mass, combine strength-training workouts with a healthy diet. Seek guidance from a medical practitioner for specific recommendations.