Read the joining package and take action as appropriate on advice given.
Buy a suit, three shirts and a couple of ties. Ask your Mum if they match.
Watch the news / read the newspaper headlines / have an opinion.
Brush up on English basics. Be able to summarise text, write essays.
Brush up on Math basics and practice so you can do simple stuff quickly.
Do Psychometric testing until your eyes hurt.
Attempt spacial awareness problems (good luck with that one).
All AIB info has been done to death on this forum - pretty much the best place to find out all the info is this thread here: http://www.rumration.co.uk/cpgn2/Forums/viewtopic/t=7213.html having done the AIB recently as an RNR DE, I can confirm it is exactly the same format as described in the link. My board consisted of 2 RNR, 1 SUY and 1 civilian entry, if that gives you anything to go by. All I will say is that RNR naval knowledge paper is the SUY one, therefore it is more detailed than the straight forward civilian entry. I assume the theory here is that you've started life as an AB and therefore you have a better knowledge of all things RN. This can be a bit tricky if you're going in as a DE. I did make this point to my AFCO so I don't know if its been looked at.