The most important preparation for coming to Cambridge involves learning to train your visual memory and to develop your own critical skills. Visit as many museums and exhibitions as you can, taking descriptive notes of what you see. Visit buildings such as churches or country houses; you may find it helpful to record them photographically. Don't be afraid to sketch, even if you have no artistic talent, as it is an excellent way of studying and remembering images, buildings and artefacts.
When a work of art or architecture excites you, try to analyse why it has this effect.
Other helpful preparation is to gain a firm grounding in iconography by improving your knowledge of the Bible and classical mythology. A good basic knowledge of European history is also extremely useful.
The course does not require any specific prior knowledge of the History of Art, but you may like to read some of the books on the reading list before you come up to Cambridge.