Hi Clairey Fairy,
The email that you should have got from NATS which said that you were invited to pick a date for the selection tests (and had a link to the online booker) also should have contained another two links - one to the document you need to revise, and another directing you to some practice tests online. The stuff that's contained in those links is all you need to get through Stage 1 (and a bit of hard work too, obviously!).
I got an automated confirmation email from NATS as soon as I'd booked my Stage 1 date with the time/date/etc on it, and this also had the same info links in it.
Bit strange that you haven't had that through - have you checked your correspondence history on the NATS website (the same place you went to start your online application)? It contains all the emails sent to you by NATS, just in case you miss one in your inbox.
If all else fails, I'd call HR (01489 616090) and double check everything with them.
As for the tests and preparation, have a good look through this forum and ask if you're still unsure - everyone's pretty friendly.
Good luck!