Welcome aboard Capitain!
Check the Newbi sticky thread at the top of this page. Lots of good info. "The Hunt" Is especially helpful in plotting intercept courses.
As for what to do. You dont have to go to your patrol area, but like was mentioned before, you do get renown for doing it. Sink anything you find or can get to on the way there. If you dont come across anything while patrolling your assigned grid then head off to a more juicy area. Some nice spots are the approaches to the English channel and on the NW of Ireland (Grid AM 51 and there-abouts). You will usually find some nice convoys in those areas. This is where I always try to go if Im not having luck anywhere else. If you happen to be way down south then the approach to Gibralter is always good.
If you are really feeling bold then try and make a run through the English channel.
Dont feel that you are locked into to anything. You can have a lot of freedom in this game.
Happy hunting Mate