How to get into top universities?

I am 16 and I am considering to study medicine in university. I understand it's a very competitive course in any university and requires high grades. What can I do to make myself stand out to universities? I was thinking of applying to Cambridge if I get A's in my AS. So, what do they expect and how can I go reach and go beyond their expectations. If you have applied to Cambridge or any other top universities, can you please lead me to the right direction. Medicine is my first option, if I don't get the grades I'm not sure what I would want to do :/