I've just done my AS levels but here's some advice from me when starting out:
-Be organised from the first month onward. Keep your notes in folders/notebooks (whichever you prefer) but make sure course content is separated into manageable chunks. End of year revision will be a lot easier if you have your notes done properly the first time round, as well as understanding content throughout the year.
-Do wider reading. For most of my subjects this really helped with my understanding, extra quotes, and creating a good work ethic.
-Make use of your frees! It is SO tempting to waste your free periods, but try and do something study-related.
-Treat mock exams with respect. That way it will give a more realistic indication of where you're at, which can be useful.