Chemistry, Biology, Maths and anything else you want to take for AS-Levels - then keep Chem, Bio and Maths for A-Level, or if you're confident enough keep the fourth too although it's not necessary. Ensure you get good grades and UMS marks for AS, get some good work experience* write a good PS and you should be in good contention, although nothing can be assured - I know someone with amazing GCSE's (all A*'s/A's), seven AS Levels at A-Grade, predictions of A*'s and A's for A-Levels and he still didn't get in.
* - By work experience I mean; voluntary work at Charity Shops, Nursing/Care Homes, Shadowing Doctors/GP's.
Good luck etc.