YOU is a very easy introduction.
The skills testing has a lot of maths and english language component. Just watch your time, read the instructions and don't space out. Its simular to a lot of IQ tests, but with less of a focus on pattern recoginition.
Oh you don't get a score either, just which jobs you can go for. I was disapointed at that fact because Im pretty sure I did pretty well, and I qualified for all positions. I also finished the test 2nd fastest in the group (one other guy got up and left with ~15mins left which seemed impossibly quick).
The medical is very easy, but be sure your BMI is okay, if your young that shoudn't be a problem, I was 5kg off my BMI limit, which I was told to lose before going back otherwise I don't get in on medical grounds. I should have dehydrated myself but was concerned it would impact on my mental test performance.
The interview portion was friendly, but no new information for me (it really isn't the place to ask any questions you don't already know the answer to). They just kind of high fived me and said sign up. I think my background story in a few short sentences told them everything they wanted to hear, for a younger recruit they might enquire about your commitment, capabilities, resilliance etc.
I also think they do a bit of observation while your waiting around, I know if I was recruiting you could tell a lot about the group. The loners, the out going personalities, the stupid. Some people couldn't read the instructions and had not started the computer test when I had finished.