I did Biology AQA and chemistry AQA.
For both subjects complete any induction work set by your college/sixth form.
For biology especially i'd look online and find the AQA textbook or some guide sorta thing and read that. You don't need to go overboard and self study this summer but it'd be useful to have a little prior knowledge before starting in september.
Also, dont buy any revision guides yet, because your college/sixth form would have them anyway and they'd sell them to you for a cheaper price.