I'm 15 as well so I think I can help you out, but then again everyones body is different. I play football (and some other sports) so I need to lift a lot to be stronger and gain weight.
Two years ago I was overweight quite a lot. I was very fat and unhealthy, but I chose to lose the weight so I shedded about 60 pounds of all fat. After this I felt I was too small to play football because I was lineman and most lineman were bigger than me.
Going into highschool football I was nervous about getting bigger and lifting. To gain weight fast it's all about compound lifts for example deadlifts, squats, bench press, press cleans, and a couple others. Do these with a weight that will seriously challenge you and you can only do about 6 reps at most. Do that with sets of 4 to 5.
Also eat chicken and vegetables all the time! It's all about the protein in ground beef and any poultry products. Also it doesn't hurt to buy protein powder, but know what you're buying before you buy it. Some are unhealthy and fattening. Do youre research on protein powders. Good luck.
Are you lactose intolerant or have any problems identified by a doctor that mean you can't have much milk?
If not, just give dairy milk a try. Soy milk is linked to a raise in estrogen and some other issues, as well as not having the same calcium and protein benefits.
aint really lactose intolerant
If the increased amount of milk does give you stomach problems, try buying a lactase enzyme pill. It just helps you digest dairy better.