Can you tell me how could a 14 year old put on weight?

Eat healthy foods: that are high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out 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, Avocadoes, granola, trail mix, dried fruits, extra virgin olive oil or canola oil.

Weight: Seek help from your doctor who in cooperation with a nutritionist can develop a proper weight program for a growing teenager