To get into Medicine anywhere you want to be aiming for the best GCSE results you can. Certain places like Oxford and Birmingham have very high GCSE requirements; Cambridge officially isn't so strict and is more interested in AS UMS, but I think the average applicant will have something like 6/7 A*s at least.
I would strongly recommend you do a lot of reading about the Cambridge medicine course as it isn't right for everyone. If, however, you do want to go for it, you will need the best GCSEs you can get, then high UMS in your ASs - you will need to take at least 3 sciences to AS for Cambridge. On top of that, you need to get all the relevant work experience and voluntary work that you need for a medicine application anywhere, and do well in the BMAT and interview.