It's been a while since I did mine, but it's fairly straightforward. There were five parts to the test:
1. Simply reading sentences from a sheet (the automated system selects three or four for you to read)
2. Reading back a sentence that's read to you by the automated system
3. Listening to a question and giving the (simple, usually one word!) answer
4. Rearranging groups of words (that are read to you) to form a fairly simple sentence.
5. Free speech - the computer asks you a more complicated question and then you have 20 seconds to give a thorough answer. It's not about getting the answer right (none of mine had a 'right' answer as such), but more about clearly conveying your thoughts. Don't worry, I didn't think I'd done very well at all on this one, but I passed!
You should receive a sheet listing the five sections above (at least, I did!). I don't think there's much you can do to prepare, other than perhaps getting someone to ask you random questions to practice the last part!