Hi wbATC.
Firstly let me say, do apply! If its what you want to do, it will be a fantastic career!
Secondly, theres no need to be anxious (easier said than done, I know). I suppose I have the benefit of knowing what to expect, but I was like you on my 1st attempt. BTW, dont get negative because I'm saying I'm on my 2nd attempt...many get through on the 1st!
Now, as far as your prep goes, theres not an awful lot you can do really. The stage 1 tests are on simple maths (speed, distance, time calculations). All tests at this stage are multiple choice. Theres one to do with shapes, and theres a diagramming one. This one has a set of 'unfolded cubes' each with a patterned side. You basically have to build it in your head, and see which of the 4 possible answers is the right one.
The other test you take at stage one is based on some material that NATS will send you once you have applied. All you have to do is learn this well, as the possible answers are very similar.
There are a number of books available. Probably best you go into a shop to look through them rather than just buying online. But you'll want to look at any that are for psychometric testing. Also, NATS send you some practice material, which is very much worth going through to get used to their format.
I know the above may sound difficult or scary, but trust me its fine! Go into the test calm and relaxed, and you should have no problem!
Good luck if you decide to go through it!
binbrain