Just chill for now, if you're dead set on doing something academically, just recap on higher level GCSE stuff for your subjects...
But, when you start, please please please work all year round!! If i could go back in time, I'd do like a couple of hours a night and about 4 hours a day on weekends, not just around exam season but all year round...
I made the mistake of doing hardly any work all year and moderately stepping up during exam season and came out with bad (however not terrible) results of BCD in Maths Physics and Geography.
What subjects are you doing?