The department provides guidance on what to do before arrival at http://www.cl.cam.ac.uk/freshers. Please be sure to read through the page and follow the advice there.
The key highlights are:
The department runs a compulsory online pre-arrival course. This typically runs from late August/early September and must be completed before arrival.
You should complete the maths workbook before you arrive.
Reading a little around the subject is useful, but there are also some skills that can be helpful to acquire (or start to acquire) before arriving. The two most useful skills are:
+ UNIX environment familiarity. The course uses both Microsoft Windows and Linux computing environments, and knowing how to use a command line interface such as that found in Linux can be an advantage. We therefore encourage students to experiment with Linux (this is made easier today using boot CDs - take a look at www.ubuntu.com as a startpoint). If you do experiment, try to concentrate on the text-based interface since knowledge of even the rudimentary command-line tools will be of most benefit.
+ Programming language exposure. The pre-arrival course gives you a reasonable grounding in Java, but it can be useful to have exposure to multiple languages, and certainly to the tools typically used by experienced developers (IDEs). For the former, consider looking at python and for the latter Eclipse, intelliJ or similar.
If you are doing the 50% options, you will also spend some of the first year studying a Natural Science subject (or psychology). If so, you look at the recommended reading for the course that you plan to take. For the latter, you should use the official workbook as preparation.