With psychology, I found that the best way to get the information was to really immerse yourself into it. It's not really like A Levels where you can just learn a list of facts. You seriously do have to read around each subject, do your own research, read news stories, and genuinely understand what you're reading. You need to get to the stage where you could hold a conversation with another person about the topic, so setting up a small study group with your friends could help with this. Make a list of all of the topics that you'll be tested on, and write key words and phrases that will jog your memory, and use these to revise.
You should also find that you get further guidance nearer the time. But go to lectures, do any recommended reading plus reading of your own, make the most of any other opportunities you get, like research groups or debates etc.
At A Level you could get away with just knowing rather than understanding, but honestly it's MUCH easier if it's just second nature to talk about.
EDIT: I agree with the past exam papers. You should also ask if they have any model answers available so you can see the depth that you need to go into. This might help to calm your nerves a little bit. An exam is never going to be as in depth as an essay because they know you can't remember everything off the top of your head.