It's never too early to start learning how to get those A*s, first, ask your teacher where they feel you stand in Maths and Science in terms of greades, and learn which exam boards you will be using. After you know this, start revising for all your science mocks and maths mocks, and frequently go over your notes and work. Nearer to exam time, start doing many past papers to know how the exam will be like. I'll be forming my GCSE revision timetable later on this year, but I do not recommend full revision now, it'll cause a burnout.