check out military.com, they have a lot of info on there for training and requirements
i was training to go into the AF pararescue which is essentially a navy seal that is a paramedic, but decided to go back to school get my degree and go in as an officer
breathing is crucial to endurance, once i figured this out i doubled my distances.
with each stroke let out a small amount of air, for freestyle it should be Right stroke Left stroke Right stoke breathe Left, Left stroke Right stroke Left stroke breathe right with each stroke exhale so that by the time you get to your last stroke you should be expelling the last of your air so you are ready to take a full breath when you breathe. takes a little practice, but after a few times you wont even think about it.
work on building your leg muscles, do some running and bike riding to change it up a little bit, if you have a pair of fins get in the pool and swim up against a wall, just put your hands out and try and "push" the wall, the fins will add a lot more resistance and really work your legs.
proper form is probably the single most important thing, im not an expert so i cant really explain exactly how, but ive been in the ocean all my life and tried swimming in high school so i know a little, take a class, read a book, get a video, do something so you know exactly how to optimize each stroke, this will be the biggest and most important part of training.
talk to as many people as you can about it get as much info so you have a good idea what you are getting into.....not sure how but i would try and prepare for "Hell week" since thats when most people drop out, the physical side is only a small part of it. everyone ive talked to said its a big mental game too. you have to REALLY want it
good luck man