Welcome abroad.
Just keep at it mate. As a tip, the convoy attack (practice mission) starts you off at a pretty good intercept angle. Not much else needs to be done other than to adjust your speed to avoid being detected. You will need to turn in to the convoy at the last minute so, just watch out for that. When I first started, I played that mission over and over again until I not only sank all the merchants, but all the DDs too. Well, actually, one merchant got away badly damaged because I ran out of ammo.
Only then did I decide that I was ready to start a career. But, that was me and everybody is different. All the advice you've already been given should light the underwater path for you. Now, go get 'em tiger.
