According to this chart:
You are of a relatively healthy weight.
Keep in mind you are still growing and if you keep a good and modest diet and a relatively healthy exercise regime (even if its basically walking to and/or from school every other day, etc...) then you should be fit as a fiddle.
Have you any interest in participating in any school related sports or say a dance class? That would be a much healthier fitness regiment then dieting at your age.
On the other hand, doing small things like drinking water instead of soda and eating celery and peanut butter, or apples, etc... instead of chips and/or cookies for snack is a great healthy lifestyle habit to get into.
I'm not saying that the occasional bit of ice cream or whatever can't hurt but the key term is moderation. Which is a simple idea and concept that a lot of people don't necessarily adopt until they are much older.
So what kind of junk food do you find yourself eating frequently? You should make a list of these foods of which you have a fondness for: McDonalds, pizza, etc... and then try and figure out healthier alternatives to choose when you have the urge to snack.