I am a 19 year old female weighing 45 kilograms and my height is 5 feet 4 inches, I want to gain weight and become 54 kilograms. What to do?

Increase your intake. More important than weight is fitness.if you feel strong and in good health why should you want to gain weight.if you eat a balanced diet and have energy you are fine.you should start lifting weights and building your bone density.that is very important for women.that will add weight too.