After i go for walks that are only a few km's, my right leg starts shaking uncontrollably, could this be due to eating to little? if not, what?

Good eating : Habits are important. I would advise a visit with your pediatrician as this is not a normal occurrence in a 15 year old. As there are other possibilities that could be causing this and not enough space here please see your doctor.

Possible : For your age, it is highly possible hypoglycemia after you walk due to not eat enough. However, you need see your family doctor.