Sometimes you don't know how to do TDD because your code isn't "test friendly" (easily testable).
Thanks to some good practices your classes can become easier to test in isolation, to achieve true unit testing.
I recently came across a blog by a Google employee, which describes how you can design your classes and methods so that they are easier to test.
Here is one of his recent talks which I recommand.
He insists on the fact that you have to separate business logic from object creation code (i.e. to avoid mixing logic with 'new' operator), by using the Dependency Injection pattern. He also explains how the Law of Demeter is important to testable code. He's mainly focused on Java code (and Guice) but his principles should apply to any language really.
