Basically what you should do is get one of the study companion book things.. like the summary books. You should know the basics of each episode. So know how the truth is manipulated or satirised etc. Know the techniques used in each. It should say it all in the book.
Also, as part of it (as mentioned above) Telling the Truth requires that you get some other texts. Use stuff like CNNN, or Backberner or the like. Something that satirises the representation of truth. Get onto the boardofstudies webpage and get the syllabus and check out what it requires. It isn't a terribly hard topic, so don't stress to much. Just work out your view on truth, make up some general introduction for the essay - "One might argue that truth is the most important thing, however the representation of this truth is that which is more important" - not exactly like that.. but you know.. Some statement that tell your view on subject (and keep it clean..)
Then just ramble on about the three texts.
Chin up, there is life after the HSC.. and believe it or not, there is like during the HSC..