If you wanna gain weight ...just eat everything in sight and lift weights..although you shouldn't be weight training at 14 since your body is still growing!!! Eat a lot of protein for lean muscle and eat a lot of carbs to gain body fat!!! try a bison burger..they have like 40grams of protein in a 1/3 lb burger..about 11g of fat!
To gain more weight fast, you need to determine how much more you require eating to gain a pound. In order to gain a pound, you will need a surplus of 3500 calories on top of your resting metabolic rate (RMR); that is, you have to take 3500 more calories than you burn.
Try to eat at often intervals in the day. You don't want to pig out to gain weight because (1)that's fat(2) you'll get addicted and become overweight. It is really hard to push yourself to eat normally after a weight gain so carefully and slowly gain weight. Don't pig on 1/2 pound burgers eight times a day.