>>715159110#1: Don't go faster than 1/2 until the rest of the fleet has a commanding lead in front of you. They are your screens against DDs and Subs
#2: Never ever show your broadside to anything. Always angle your hull (to the best of your ability) so that any incoming fire will have at least a 35 degree wall to bounce harmlessly off of. Your biggest concern is other Battleships sniping you, just as you are a threat to other Battleships.
#3: It is a general rule of thumb (but almost never a bad idea to ignore) to not "punch up" or "punch down" on ships bigger or smaller than you. Cruisers firing on Battleships will likely do nothing to them (unless they have HE shells which will light them on fire). Battleships firing on Cruisers will do minimal damage if firing AP shells since they will just over penetrate.
#4: Destroyers and Subs will almost always get the drop on you unless you position your ship around friendly ships appropriately. Even with proper positioning though, you still will most likely take more than a few torpedo hits
All of these are GENERAL rules of thumb. They are not baked-in rules that "ought not ever be broken", but are good things to keep in mind.
To get a bit more gnitty gritty in detail though. Every hull has a "Citadel" where all the main pieces of the ship are: Engines, Magazines, Crew, etc... The idea of the citadel in this game is to be very difficult to penetrate EFFECTIVELY as you want your shells to detonate inside the Citadel and not just punch straight through it. Too much penetration will cause the shell to just pierce through it and do basically nothing beyond superficial damage, too little penetration and your shell will just bounce off it to the same effect. This is why Battleships shouldn't use AP shells against Cruisers (because their shells will pierce through), and why Cruisers shouldn't use AP against Battleships (their AP shells will just bounce).