Welcome aboard
Good place to find single ships traveling alone is the Irish Sea. Pretty much anywhere in AM 5x you'll find some lone ships or a few in pairs. Then continue west once you hit AM 51, which will take you north of Ireland, a lot of convoys will comy through there. Since you're right in the heart of enemy territory beware of planes, they can easily sink your sub with a good hit or at least leave you with a real bloody nose if you don't dive soon enough (if you choose to engage aircraft, generally best to let the AI crewman do the shooting).
Another great area to sit and let the targets come to you is Gibraltor; you don't need to even enter the actual strait in order to find plentiful targets. CG 95 & CG 94 are high traffic areas, if you want to go fishing around there it's one of the few times that using the search pattern (or creating you own) will pay off. Many convoys' destination is Gibraltor so frequently will run into an entire convoy just a square or two out.
In the Northern approaches and farther West you'll frequently get reports of multiple convoys in your area, there might be two heading in different directions or perpendicular courses to each other. When possible, go after the convoy travelling slower (which will usually be around 5-6 knots). It makes it easier to catch up and intercept the convoy and can shadow them fairly easy if you want to wait until nightfall. The faster convoys usually move at 10-12 knots; which if you are able to plot an intercept course and make them come to you its no problem. Trying to catch up to a fast moving convoy can burn up a lot of fuel fast having to travel at full speed or faster depending on how far away it was when last sighted.