UK20,
I assume you can't actually book a stage 1 slot until your eyesight is given the okay. Once you are able to book a slot ensure you give yourself enough time to practice the test material and learn the motivational information, there's no sense rushing it and booking into the first possible slot only to end up failing because of lack of practice.
There's little point in thinking about stage 2 yet because it's really not something you can prepare for, at least it wasn't when I did it a few months ago. It is a near enough a whole day of computer tests, the only familiar part of the test was a few cubes.
I bought a one of those psychometric test books for stage 1 aswell, it was useful.