For those trying to gain weight, don't suddenly force large amount of food into your stomach. You need to give your body time to adjust. Increase your food consumption in stages (Health Food I mean).

And just as important, you need to go to the Gym, to sitmulate your body and muscles to grow. Without the correct stimulation, you are just trying to put on mainly Fat weight around the waist.

Also a third factor, when you start working out at the Gym, you also need a lot of rest and proper sleep. That's when your Body repairs and grows from the nutrients derived from the food that you eat.