I recommend creating a mission with the torpedo plane of your choice, and hitting a first stationary ships, then moving. Start out with Cargo ships at first. (There is already excellent missions for B5N2 in single missions.)
Also can press tab to see the parameters. ***Note-- for Japanese planes, if not using the speed bar, remember the Japanese instruments are in knots indicated airspeed (KIAS) and pressing tab will give you Kilometer per hour (KPH). So conversion is necessary
If you do use speed bar, use the toggle speed bar key to cycle to correct KIAS display.
But just keep practicing, and after you can hit cargo ships, try going against something that will fire back.
And you can get with me, Dolphin, on hyperlobby if you want to have someone help you. I enjoy torpedo missions.