I weigh 120 and im 5'7 how can i gain weight and still eat healthy?

BMI: At that height and weight your bmi calculates to 18.8, which is within the normal range. You do have room to gain weight and remain normal, though. Eat a diet balanced in the four food groups with increased calories and you will gain weight.

More protein: Have some labs make sure there is no medical condition that can lead to being underweight. You want to aim for around 140-150. You need to increase protein in your diet. Good luck.