Your weight depends on your age, your gender, genetics, and body type. If you've tried everything to gain weight - and listened to all the tips friends tried to give you (eat a lot of protein, sugar, and carbs, follow a strict fast food diet for 2 months, eat right before you go to bed, eat more snacks, eat four meals a day, ETC) but still have not gained a single pound, maybe you should stop focusing on your weight.
Instead, focus on your health. You are the owner of your body, so you are responsible of taking care of it. Dumping junk food in your body just to gain a couple of unhealthy pounds does not sound very caring, does it?
A healthy way to gain weight is by, believe it or not, exercising. And I'm not talking about the treadmill - I'm talking about weight lifting. Physical Therapist Ajith Jain says:
Strength training is recommended for gaining weight. Taking good nutritious food, high in protein, would help your muscle gain hypertrophy and increase your body composition.
If you try to gain muscle instead of fat, you actually control where you want to gain weight. For example, if you would like your arms to be a little fuller, you lift weights with your arms until you've build muscles.
A lot of girls and women are afraid they are going to look like a 'muscular boy', but for most women, it's not even possible to gain that much muscles. Women develop muscles in a different way than men do. You really won't look like a oily muscular weight-champion when you gain a couple of healthy pounds by weight lifting.
A good way to gain muscles all over your body is by rock climbing. During rock climbing, you will use almost every single muscle in your body. Your legs, arms, tummy, hands, - everything will be used, and therefore all those muscles will grow.
So please, dump the junk food - and start eating some banana's! Be proud of your body and who you are.