Sambo
I sat my AIB in '95 and would thoroughly recommend a booklet the RN provided called Ships, Guns and Missiles' or something similar. The latest version may be a little bit thinner though
Know the different weapons platforms and their capabilities; where the RN are operating; and what the first 3 years of you career will entail. The map of the World without country names is a good one.
Try and organise a visit through your recruitment office. I was fortunate to spend 9 days aboard HMS Dulverton - a Hunt Class Minesweeper - on a foreign visit and had an absolutely fantastic time.
Relax, be yourself, enjoy the experience and Good Luck!