Welcome aboard!
If you're gonna play a simulation, a naval one is easy one to start with. Unlike a flight sim you don't need a joystick or to learn how to take off, or land or fancy aerodynamics. The boat just floats, dives and drives in the direction you point it. The manual torpedo targeting option can involve a bit of geometry, but you can also let the 'crew' handle that one. These games also have some basic schooling courses that teach you navigation, gunnery, torpedo firing, etc. When you sneak up on a target and deliver your first torpedo salvo you'll be hooked for life...most likely.
Preference for SH3 or SH4 really comes down to which theater interests you the most? The Atlantic with the Germans, or the Pacific with the Americans. However, SH4 is the more recent release so the graphics for it are a bit more up to date.