How to start unit-test old and new code?

Question

I admit that I have almost none experience of unittesting. I did a try with DUnit a while ago but gave up because there was so many dependencies between classes in my applicatio

Answer 1

For .Net unittesting read this : "The Art of Unit Testing: with Examples in .NET"

About best pratices :
What you said is right : Sometimes, it's difficult to write unit tests because of the dependancy between classes... So write unit tests just after or just before ;-) the implementation of the classes. Like this, if you have some difficulties to write the tests, maybe it means you have a design problem !