I've done this a lot in my career. My strategy is simple: start by finding a trivial bug, and fix it. Then find a slightly less trivial bug. Do this four or five times. Don't ask anybody about anything unless I'm really lost, just chase the thread and let the bugs lead me through the active code paths.

After doing a few of these I've got a fair understanding of the structure of the code and can figure out where to go next.