This is a re-post of one from late 2007. I was in at a local school last week, talking with their scholarship candidates, & we talked about a lot of this stuff. So I thought it would be worthwhile to post it again (at the top of the queue, so to speak!) so as to catch your attention :-)
... Remember that the examiner will expect you to be familiar with the Level 3 content that you've studied this year. They may expect you to apply that content knowledge in a different context, and we'll come to that in a minute.
Have another look at previous years' exam papers. There are the same three 'big ideas' (or basic principles) that crop up again & again: genetics, evolution, ecology. So it's a fair bet that they'll be there again this year.
Sit down with your friends & brainstorm all the concepts & ideas that you can come up with that are related to those 3 principles. See? You do already know a lot of this stuff.
Now have a look at your list (you might have put it into the form of a mind-map or a concept map) & see how many of the items are related to each other & to different principles. This is a good thing to do as it will help you to integrate ideas - & this, of course, is very definitely something that the examiner is looking for (have a look at the Schol performance standard).
Choose a question from a previous paper - & read it carefully. What's the basic principle/big idea that's central to the question? And you'll find that there are also 2-3 key points, concepts, or ideas that show the scope of the question &/or give some idea of the amount of detail that's required. For example, question 2 in the 2006 paper asked you to discuss the genetics of inheritance (that's the central 'big idea') of the alleles for A1 & A2 proteins in milk. The examiner asked you to discuss how the alleles arose, and the pattern of inheritance shown by these alleles, and to give an explanation for their relative frequency in the gene pool - this gave the scope of the question. So the context for the question might well have been unfamiliar (the A1/A2 milk), but you could apply knowledge you already have, in answering it.
In fact, it's very likely that you'll be given a context that you haven't studied in school. Don't let that throw you - the concepts will be familiar. And remember to read the contextual information carefully, because you should use that in developing your answer, relating it to the concepts that you already know.
And finally - before you start writing your answer, take some time to plan it. Use a concept map, a mind-map, or whatever works for you, it doesn't matter, but write that plan. This way you're less likely to waffle, more likely to include only relevant material, and you'll be able to organise your answer into an analytical, coherent, and integrated response to the question.