Where are you at? I would try to avoid learning what's next at school. It would probably improve your grades but will make you feel bored which is never a good idea when learning something. Of course you can always read books on night skies or planets. Wikipedia alone offers many data at no extra costs, and the scientific part is usually pretty good, i.e. not biased. At least as physics and mathematics is concerned. There are no hidden interests like in their political or pharmaceutical sections. Nobody wants to cheat about the life cycle of stars or the properties of functions.
To reach your goals, you will need a mathematical foundation. Usually trigonometry, polynomial functions, differentiation and integration will be part of lessons at school, at least their basics. Without crossing the usual curriculum too much, I would suggest to learn some linear algebra, maybe a bit topology, and add further contents to what you will learn at school. This way you can get the foundations needed, and simultaneously extend them to a more advanced level. My suggestion would be to come back and ask the guys here, what those extensions are corresponding to your regular stuff. PF is also a good source when it comes to the often hidden fundamental rules and mechanisms behind the exercises.
The best advise is always: Stay curious! At your age you probably still have the luxury to simply learn what interests you the most.
