Welcome aboard
On to targeting whilst submergerd, the only way is through the use of you attack scope (or sonar, if your U-boat is equiped with it. don't worry about that yet.), and by plotting your targets course/intercept course on the map.
As mentioned before, you can find various links to how to do this by searcing the forums.
And if you don't get it first time, just keep at it and you will be rewarded with a successful kill(s).