What to do if I'm 20 years old, 5 foot 5 inches, and I weigh 89 pounds how can I gain weight?

Consider working w. a nutritionist. Eat 6 meals/day. Work on strength training to build your large muscle groups. Workout w heavier weights w fewer repetitions. Sleep at least 8 to 9 hrs/night. Hydrate well w water. Foods to help w healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, Avocados, granola, trail mix, dried fruits, extra virgin olive oil or, canola oil.