What Max and Bazza said.
I wouldn't bother doing cardio. At your age, your calorie requirement is still so high that you can get away without doing any cardio.
I think the only reason anyone needs to do cardio to lose fat is if they are at a point in their life where their calorie requirement has dropped so much that to diet WITHOUT cardio would require say 1500 calories, or something ridiculous.
Eg, I am currently losing weight (perhaps too much) on 2700 calories without worrying about cardio. 2700 calories is plenty (ie I'm not hungry), so I don't bother with the cardio.
If I was on say 2000 calories and struggilng to lose weight, but I couldn't eat any less food cos I was starving, then I'd keep the calories constant, but start doing some cardio.
That's my understanding anyway. It is always a trade off between calories and cardio. So if you want to eat more, do more cardio.
Or if you want to improve cardio fitness, but I am of the view that gym training builds a sufficient level of fitness to not have to worry about it.