Yeah Bhootnike's right.
Write examples of the types of essays that have come up in past years of vcaa papers, popular one's have included (from the top of my head):
1) Language change across time
2) Language and culture
3) Euphemism (incl. doublespeak) / dysphemism
4) Standard english and non standard english
5) Language as a way of conveying identity
6) Language and technology
6) Spoken and written english
Make sure these are all relevant to the current study design though.
Pick out maybe 4 out of 6 of the topics that you are strong with, and work with those ones, usually at least one of them will come up in the exam! There's only ever been one time where 2 topics have been given for the essay, usually there are 3.
Find quotes, maybe 2 for each one and analyse them, these will form the body of your essay and give evidence for your argument, you may also like to find one's that you do not agree with (probably will get you extra marks). You are required to use AT LEAST ONE of the examples they provide you in the prompt, but using 3 of those will not score high because it shows you haven't adequately researched the essay topic.
Also make sure to refer to australian english whenever you can in your examples.