The first step would be to read the bug description from Bugzilla(or whichever defect management tool you use). Once you have read the bug description, you will know exactly what is misbehaving.
Once you know the feature that is misbehaving, there are two ways you can go ahead:
1. Try to reproduce the problem using the steps in Bugzilla. A good tester will mention the steps used to generate the problem. If it is easily reproducible, then tracking the defect can be done by either using a debugger or by code inspection.
2. If it is not possible to reproduce the problem, then code inspection will be your only way out. Typically, some problems like thread synchronization issues, deadlocks etc. are not very easy to reproduce. Only a very thorough code inspection will lead you to the problem.
Of course, the one basic thing you need to know is the code flow. You must know which classes and methods are being invoked for the functionality against which the bug is reported.