Ask HN: TDD Resources for beginners?

I am eager to dive into the world of TDD (I am primarily a JS/PHP developer). I have read about various frameworks for unit and integration testing but still feel like I am missing something.

The biggest question I have is exactly what the scope of the tests should be. I don't want to test too little, because that would largely defeat the purpose, but also don't want to test every conceivable thing. Are there best practices for knowing where to draw the line or resources out there that will point me in the right direction?