You know how you manually test code sometimes in a console, like a REPL?

Automate that and you have a unit test.

Do it right before you write the implementation (forcing you to first consider how it might work, not a terrible exercise), and you have TDD.

Every unit of code should be smallish, focused, and with the absolute minimum of external dependencies. That will make it easily unit-testable and far more maintainable long-term.

There's finally empirical data emerging that TDD is a labor-saving device... Look up the Nagappan paper. (There's more than that now, though, that's just the most famous one.)

Also, as someone else mentioned, watch the Boundaries talk by Gary Bernhardt. Amazing.