The best place to start is think of every prac you've done this term and revise them. If this is too daunting, don't worry, just go though the points on your syllibus and find something major or your teacher spent time on with the theory side of it. This is normally the prac they choose as they need to give you something that would likely be in the HSC examinations.
They don't tell you what prac you will be tested on because otherwise it will be too much like an open book test where you cram the night before and if you've taken good notes, you'll get 100%