I've been doing alot of the sub service screenings out here on the west coast - the ones they are getting early on are usually electricians and sonar ops. Not sure if they want to start doing that at the QL3 or 5 level for the NCI Ops. That doesn't preclude volunteering for sub service though, especially at an early age. I'm sure someone will be wringing their hands with delight to have that form come across their desks, and a bunch others breathing a sigh of relief (alot of voluntolds for the sub service right now and most don't like it at all).
Sorry I can't be more general than that - once you're in training, approach your chain of command and volunteer for submarines - they'll give you a briefing and a screeening sheet to go through to make sure you're administratively and medically suited for it. If all goes well, once you're at the Qualification Level they want for your MOSID, they'll load you on BSQ and so on.
Cheers and good luck.