Allow me to whole-heartedly recommend Legion's Noob Guide! It addresses this very issue, an issue I was having as well. And I find his to be less difficult that the advice posted by Zero Niner because you don't have to know exactly where he's going to be.
Link follows:
http://www.subsim.com/radioroom/show...ons+noob+guide
If you don't have time to watch the video, here's a quick run-down.
1) Draw a line along the targets projected course. It can be as long as you like. As you use the method more, you'll get a feel for exactly how long to draw it. For now, over-estimate and draw it long.
2) Use the compass to draw 2 circles ~10 nm in diameter, one at each end of the line and then draw 2 lines tangential to both circles (touching the edges of the circles without passing through them in case you've forgotten High School Trig) about the same length as your first line. It doesn't have to be precise, just close. This is what he calls a "probability corridor". Unless the target changes course, he will be somewhere in here.
3)Now, just plot a course for your sub to the far end of the corridor and then plot a course that zig zags up the corridor towards the target's last known position.
4)Assuming you gave the target sufficient lead time (IE he didn't leave the corridor before you arrived on station to begin your search) and he hasn't changed course, you cannot help but to make contact with him. Before 1943 and the installation of SJ radar on your boat, you will find it helpful to dip to about 10 feet below periscope depth periodically to check for hydrophone contacts. I like to do this when I'm at the turn in my zig-zap pattern, but you can do it whenever.
5)Once you find them, plot a course that takes you to either their port or starboard side so you can be set up for your shot.
The actual guide has a lot more info in it and carries through the intercept, the approach, right up to the kill. Quick note: Legion uses one mod in particular, the 3000 yard bearing tool, that may well be the most useful mod you download. This is located in the downloads section of this site.