If you want to get into Cambridge you should be looking at 10 or more GCSEs, in traditionally academic subjects, with at least 5-8 As depending on where you have been to school, and whether you are applying via the special access scheme, plus something like 2 AS levels with As, and 3 A Levels with AAA predicted.
If you've been offered a provisional place, you'll most likely still be allowed to come if you get AAB due to some sort of minor problem in one module or something, and once I saw an ABB entrant let in who did fine when there.
You should be used to having academic debates with intelligent peers - if you're at a school where there aren't too many other people around operating at your intellectual level, it would make sense to start investigating summer schools and so on.
You also need to be quite self-starting and resilient, the type of person who is happy to get up at 8 to get to the Faculty library when it opens, and spend all day reading hard books because you are dying to know more about the subject. It's also important to be able to find a hobby or two otherwise you would probably go nuts.
A really good guide to what is expected is gained from looking at the reading lists, set essays, past papers and so on, many of which are on the website. The book "How to get a first" is a great thing to read as well, if you want to get inside the university mindset.
It's not rocket surgery, as one of my colleagues frequently says. 