I am new to studentroom, so I am not that familiar with how it works!
For Biology and Chemistry, make a list of AS/A-Level papers by year, and tick them off as you do them. This most likely won't be done at the start of the year, but nearer to mocks it will be useful. Try to actually revise for your class tests, because most schools take a holistic approach to predicted grades (it won't just be your mocks, but tests as well). Also, if you flop your mocks (fingers crossed, you won't!), your teachers will know its probably an one-off thing.
Use your frees to complete your homework, so you'll only have to do revision at home (or vice-versa)
You'll probably have a lab/practical book - just try to keep on top of it, my teachers were quite picky on that.
Just do as many exam questions as you can, as well as questions in your textbook.
http://www.pxsbiology.com/ - has some great exam questions.
Tips for medicine in general:
Try and get work experience sorted as soon as you can. Ideally start looking for places now. I'd suggest 1 week in an hospital and GP. (Work exp, is hard to get, so volunteering can be used to supplement)
For volunteering, visit your local care home/charity shop on a weekly basis - 6 months should suffice
Try mentoring/tutoring and stuff in school - ticks off the roles and responsibilities skill that unis love to see!
Try and apply for summer schools and stuff as well - UNIQ, Suttons trust etc.
I don't know if you live near London, but KCL have 'Medicine in action' lectures which might prove useful - other unis may have lectures/workshops, which you might wanna research.
Remember to do your UCAT/BMAT during the summer, as well as the personal statement.
I suggest you get as much of the work exp and volunteering done in year 12 - so you can purely focus on academics in year 13.
Good luck!