That really depends on you, and the language you want to learn in.
One resource I recently stumbled over is Test with Go. While Go may not be your preferred language, this course does a quite decent job of introducing test concepts, and thanks to how Go works, you also get to see both the good and the ugly sides of testing.
For getting into the spirit of TDD, I would recommend researching the topic of code katas. It is basically you repeatedly implementing a generic thing in a language of your choice using test driven methods. Many cities have local code kata groups, which I would recommend as TDD works best when challenged by another person.
E.g. having a colleague to implement code you wrote tests for and switching with each iteration is a great experience.
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