This guide came about from trying to coach a family member in the basic concepts of aiming/gunnery used in this game.
First thing you may notice is that I use a crosshair mod because I find the one supplied by the game to be hopelessly inadequate. This mod package has several crosshair mods so you should be able to find one to your liking. The one that I am using is an older version by Hooterez.
Above is a cruiser that is not targeted.
This is a destroyer that is targeted. Notice the white circle around the red destroyer symbol. This white symbol is sometimes referred to as the Celtic Cross. It designates the target that your plotting room is actually taking note of. Alternately it is the target that you are locked onto by range also known as target lock. Of note, you cannot lock onto a ship unless it is within your gun's range. Your gun's range is signified by a dotted circle drawn around your ship on the mini-map.
You establish target lock by aiming directly at the ship for a couple seconds or by aiming at it and typing X.
For a little more information, notice the trail of smoke coming from its smokestacks. They can actually tell you what direction the ship is moving and with experience you can get a feeling for how fast the ship is moving too.
Hint -> zoom in to target by hitting the shift key. For more fun, scroll the mouse button to get an even closer view.
This is the mini-map. It is normally located at the lower right corner of your screen. In this image you will notice gunnery range as signified by the dotted circle in the upper left corner of the mini-map. You may also notice a grey cone emanating from the ship. This is the view range. Anything you can actually see will be inside this grey cone.
In this image a destroyer is targeted and notice my aiming point (the yellow marker on the far right). The ship is relatively close, but there is insufficient lead so the shells are falling behind it. Notice the splash and yellow tracers going to the far left. This means I should move my aiming point to the right, approximate 6 numbered ticks on my aiming bar. Did you notice the numbers on this particular aiming crosshair? For now aim for the center of the ship. With practice you will be intending your shots to fall under the smokestacks or other designated critical areas.
In this image, notice the shot (the orange glow) is below the ship and ahead of it. I need to adjust my aim at least 3 numbers to the left and up!! Try to get the long/horizontal bar from the crosshair on the waterline of your target. Later you can aim higher or lower as experience dictates. The short form of this recommendation is . . . if the shot falls short, raise the targeting mark.
Here we see the shot going well beyond the target . . . lower the crosshair. (yes there's also too much lead, but needed extra lead for the image)
This is a nicely complex image. The targeted ship is the one on the left. Notice the Celtic Cross. Also notice that ship is moving to the left but I targeted far to the right. You guessed it. Shot behind the ship and far too low.
The last part that's harder to show in images is . . . a turning target will require more lead if it is turning towards being parallel to you and less lead if turning towards being perpendicular to you.
With time and practice you can land even your first volley on target. Have patience. Practice every day, even if its only 1 or 2 battles. Somehow your subconscious will process the information and you will improve.
For Advanced Aiming/Where to Shoot, consider this excellent guide http://forum.worldofwarships.com/index.php?/topic/32522-tedsters-updated-and-expanded-where-to-shoot-guide/