Okay .. serious now. I admit I don't know what the army requirements are or how the course is structured but one thing I'd recommend that you do (from my experience in another field) if you get as far as the selection process... do get comfortable as you can with a blacked out face mask!!
Get a cheap old scuba mask, put gaffer tape over the face plate, and get used to the feeling! It doesn't sound like much and if you are on dry land and in familiar surroundings you may just feel a bit daft walking with a blacked out mask on (make sure you have a mate keeping any eye on you) But when you are several metres underwater and you are 'one-breath-from-death' it can be quite unnerving.
You need to talk to your unit's Diving Officer. He can book a Diving Aptitute, a course of about a week. Medical, and phyisical tests included. If you pass the aptitude it lasts for either 6 or 12 months. If you don't do the course in the required time then you have to do it again. The schools in Portsmouth - quality on the piss.