This is probably a stupid question but my googling isn't finding a satisfactory answer. I'm starting a small project in C#, with just a business layer and a data access layer - strangely, the UI will come later, and I have very little (read:no) concept / control over what it will look like.

I would like to try TDD for this project. I'm using Visual Studio 2008 (soon to be 2010), I have ReSharper 5, and nUnit.

Again, I want to do Test-Driven Development, but not necessarily the entire XP system. My question is - when and where do I write the first unit test?

Do I only test logic before I write it, or do I test everything? It seems counter-productive to test things that have no reason to fail (auto-properties, empty constructors)...but it seems like the "No new code without a failing test" maxim requires this.

Links or references are fine (but preferably to online resources, not books - I would like to get started ASAP).

Thanks in advance for any guidance!