You're at the age where your body rapidly loses weight. Eating junk food will not help, because empty calories will negatively effect your energy and metabolism. Incorporate protein in your meals, like beef, chicken, pork, or fish. If you don't eat meat, you can use nuts, seeds, and non fat greek yogurt for example. Find what's best for you appetite and make sure you do not eat too much just to gain weight. That's not how people your age gain weight.
Hope I Helped ^_^