You should receive contact reports in regular intervals from the HQ, which will also mark them in red on your map, so watch your radio messages received, they also appear in the Message box. Then draw the approximate route with the ruler and see if you can intercept the convoy or the task force. Once intercepeted, you can send a contact report yourself (it's the ship icon in your objective log), this will work once you are on the surface and have 3 ships in sight, in this case sending this report *can* make the HQ send you air support if friendly ariplanes are in range, and it should also move friendly units towards your position.
Other than received contact reports, you need to find ships for yourself. For doing it there are different strategies, I would not move much with my sub because it depletes my fuel, I would position on 'logical' routes which are routes between ports, important straits where ships must pass etc. Later in the war your radar and sonar ranges will increase, and so you get more contacts, but early in the war this is more difficult like it was in real life for submariners. You have the big advantage that you know about the 'historical' routes that you can follow and which most likely will yield interesting encounters; real skippers only seldom had this information.
You don't need to stay in your operation area delimited by the mission icon on the map, you can go further away, but generally the probability of finding contacts near the mission marker is increased. It's all about tonnage, so sink merchands everywhere you find them. Once your mission is 'Completed', sending a status report will give you new orders with a new operation area.
I hope this helps.
Greetings,
XS