Welcome aboard. As has been noted, there are quite a few free to download mods for SH3, ranging from simple tweeks in how your sub looks, to major facelifts and changes to how the sim plays. Like most things, you get better at them with a bit of practice and end up finding the style of play you prefer. When you know that, you'll know whether to stick with just a few tweeks, or add on some real hardcore mods to make life really tough. But most important of all is to have fun.
And if it is advice you are looking for, short of going back in time and joining the Kriegsmarine, this is the best place for it.
Chock