you can try some of those shakes that they have in GNC or the Vitamin Store, they help build mass and add weight. You can also try to add a bit more protein to your diet. Muscle weighs more than fat, so if you work out and eat a balanced diet rich in protein, you'll get the right kind of weight. You can also add some Ensure along with your other meals to help give your body a bit more nutrition. Some people are just just naturally skinny due to their metabolism, and even if they eat a lot, or eat a high protein diet, it's no guarantee that you'll gain any weight.
You should just be happy with your body for now. Believe me there will come a day when you will want to lose not gain. So enjoy and take lots of pictures so people will believe you were very thin when you were younger because they usually don't.
Actually eating unhealthy food and bigger portions won't help you if you have a high metabolism like I do. You should try some weight training and gain muscle to help fill you out. Plus that's a much healthier approach that you won't regret later in life.