Stick with one sub at first. Pick the 688 or the Akula but then stick with it while you gain experience. Learn where your boat cavitates for a certain speed and depth. Then, get used to the different stations. Trust your auto crew at first then learn when its best to have them turned on or off (its really up to you, some players like the auto crew, some don't, many use them at the proper time as they see it). Save the Kilo for last. It has some advantages but also a lot of draw backs. Good Kilo skippers tend to have a lot of experience.
Play through some single missions at first. There's a deep secret that many of us will replay a mission
many times before we get it right.
The stock campaigns tend to use all of the things you learned in single missions.
Before you switch to a different class of sub, spend some time in the helo, P-3, and FFG. Learn what they're good at and where they are limited.
Above all, give yourself some time. Many of us learned by making mistakes, sometimes making
many mistakes.
There is no single,
correct way to play DW or the later RA mod.
There's also a good community here that's ready to help you.
If you have questions, just ask.