Stop stimulating your brain with computer, tv or video games at least an hour before you go to bed. No sugary or energy drinks either. Try a warm bath or warm milk. At 13 you shouldn't have problems falling g asleep.
Lay down and close your eyes, get comfortable and take 16 deep breaths and start relaxing from the top of your head all the way to your toes like first your head then your face and jaw then shoulders then arms... Keep going till u reach the toes