You're in 8th grade...
You'll be getting better scores just by getting older, let alone studying, so don't worry about it.
I'd say the summer between 9-10 grade is even an early time to start, but of course, it's up to you. If you REALLY want to study, get Direct Hits I and II, Gruber's Math, and study some grammar on Sparknotes. Don't let it take up too much of your time though, you still have 3 YEARS.