The most important thing to do is to go fast and shallow.
It makes for an exciting, albeit short, mission
Seriously. I hope you will really enjoy yourself. As a Flight simmer and a Sub driver I find the two as far apart as can be and I enjoy them both for what they uniquely offer.
There are a ton of great people, Subguru being one of the best, on this and a number of related forums. Very helpful and willing to teach anyone nearly anything about naval warfare. They've certainly helped me a bazillion times.
Check out the Bluebook and the TMA guides.
Hope to see you on a dive, if you decide to move ahead with DW.