Nutritional Changes for someone under 18

I got the MyFitnessPal iPhone app and really wanted to go on a runner's diet and make an effort to eat healthier. I'm 15 and you have to be 18 to join. So I thought "oh, no big deal I'll just say I'm 18." I understand why you should be 18 to a diet plan site (or calorie counting site. call it what you will). Because you're pretty much done growing at 18. I'm on a maintaining weight diet of 1900 calories a day and I often go under that. How should I change my diet (nutritionally; like fats, cholestoral, sodium, carbs, sugar, fiber, vitamins, calcium, and iron) because I'm still growing.

About Me:

I'm 103 lbs. Anywhere from 5'2" to 5'5". I'm underweight according to the BMI calculator but my body's not toned. I'm trying to lose that stubborn thin layer of fat and gain muscle, maintaining my weight. I lift weights 5 days a week. On one day off I stretch and the other day off I just do push ups and ab exercises (though I do that the other 5 days anyway, but that day I just go harder on my abs). I'm a track and cross country runner, but winter track just ended and spring track's not for another month. I run on the treadmill everyday. I burn 300-500 calories a day on that. For the most part, I run 15-20 minutes at a time. I'm trying to get up to 40 minutes, but with all the heat indoors, it's near impossible unless I jog slowly. Running outside I can do 1 hour no problem, but it's too cold for outside and if I bundle up its too hot and awkward.