research the STEP. make sure you dont underestimate the time it takes to prepare, the difficulty, etc..
i used my summer after yr11 to reaad a few maths books (recreational, not theory). well that was all the maths prep I did. good to just chill a bit and go hardcore later on.
I can recommend a few books if you want, I read like 10 overall, haha. stuck about 6 on my personal statement and did actually get asked about quite a few at interview.
basically, best read books and such to motivate yourself and just "know" maths better ya know. going to the biggest sixth form in the country, I saw about 15 or 16 people at least apply for cambridge maths. 2 (including me) got offers, and it was pretty clear why. I reaad a whole bunch of books, and I did more than just what the curriculum expected. the same goes for the girl that got an offer too. being very good at maths (and willing to work hard) is a prerequisite, but its not a sufficient condition to get an offer.
feel free to pm me if you need any specific advice or anything budd.