Alfred is right, but the fact that you're 13 concerns me. Most every bottle of Melatonin that I've seen has had a warning that if you are under 16 (or 18 on some of them), you should ask your doctor if it's safe for you to take... the reason is because Melatonin is a hormone and in your teens, your body is already trying to balance out the hormones that are already in a delicate flux that adding a bunch of other hormone sources can just throw the rest of the hormones in that balance out of balance.
Ultimately, I wouldn't recommend taking it at all without your doctor's okay. Until then, you could try a Diphenhydramine (same stuff that's in Benadryl or Tylenol PM) when you just can't sleep. Also, make sure you avoid sugar, caffeine, and exercise for at least 2-3 hours before you want to head to bed to eliminate other common possibilities of sleeping problems. If none of those help, it's possible you may need to see a Sleep Specialist to see why you aren't sleeping right, but most people don't get to that point after trying some of these options. Good luck and I hope I helped!
Personal experience and personal research in my role as a Product Specialist for a vitamin company