I am 19-year-old female. my height 158 cm weight 39 kg bmi 15.89. i eat a ton everytime but still skinny. i wnna build mass muscles. wht should i do?

Medical evaluation: You need to make sure you don't have a medical reason for not absorbing the food(proteins and fats) you consume. You may have a high metabolism in addition, for which your thyroid function or diabetes etc need to be checked. Once you rule medical issues, you can work to develop a plan to start regular,are exercise and protein supplementation to gain weight. Good luck!