Answer:
1 - Get some solid A-levels, you need a minimum average of 85% if you ever hope to get to Cambridge, an A-level in Maths helps a lot too.
2 - Get a golden medal or an outstanding achievement in something like a national Olympiad or some well known competition or test.
3 - Write an awesome personal statement, tell them you are interested in quite a lot of fields, and you have way too many talents, while keeping the modesty.
4 - Bring great recommendation letters that doesn't just say you are so good, but rather you are the best they've had in a long time.
5 - Impress the folks in the interview, be very honest and interactive, the interviewers are the ones who will teach you later so they will look for someone they'll enjoy teaching.