Everywhere on the Internet you can find articles about “how to use junit” or “make your first unit test” but none about “Which approach to take to learn TDD”.
I think that following a tutorial that teaches you how to make a unit test without knowing when, where and how use it, it's the wrong approach. So I spent some time building a path, a road to follow.
This arguments/resources are in order, meaning that I found (maybe later at my own expenses) that learning about these topic in this order is the best approach I could take.
Learn about Seams (not the Framework but the concept):
If you have access to this book “Working Effectively with Legacy Code” by Micheal C. Feathers in a near library there is a good passage with an explanation about them. But I think you can easily find some info online.
Learn about The Factory Patterns:
You should know at least this two patterns to follow how the instantiation code need to be separated from the business code.