I'm underweight!!?!

I'm underweight!!?

I'm about 10-20 pounds under weight.. I'm not anorexic or anything, it's just that i'm a bit too skinny for my age since i'm at the puberty stage.
How can I gain weight? I've heard of drinking a milkshake a day, but what else?

Keep a balanced diet and do exercise. Your muscle tone will make you look better, and muscle is heavier than fat, so you'll gain some weight. On the plus side you'll have a perfect health, and your skin should look better too. Don't eat sugar and fat. Sure that will make you gain weight, but it will ruin your health and your looks.