Hi and welcome!
From what I understand, you need an overall introduction @ finding a target.
That is to say, once you found one, you're in for the real deal...but that's of a later concern. :P
I don't know if your game came with a nice map of the N-Atlanctic, so
here is the one from SH3.
You can see big shipping-lanes, where lot's of traffic can be expected.
Zoom-in on your map, and see to which individual harbors the large lines might split-up
You can choose the vital lines to the US and Russia (Moermansk), or go for the more coastal traffic - the English East-coast comes to mind.
Also, take note of the big circles on that map...those indicate air-cover at a certain stage of the war.
Once you're inside such circle (depends on which year you are playing), prepair to get sighted during the day.
So make your speedy progress during the night.
But don't forget to dive - go to let's say 25m - every now and then, run-silent, and listen to the hydrophone.
Warships have high rpm-sounds, merchants low.
Suppose you finally find a contact, don't fire untill you can see which flag the vessel is carrying!
Good hunting.
*edit - just found out, that
reaper7 made a printable SH5-map....guess it's more relevant to SH5's "campaign".