What to test when using TDD and asserts

First, I'm a newbie at both TDD and Python. I don't code for a living, but I do write a ton of code.

I'm in the process of moving a large project from Matlab to Python. 1) because the language has been limiting, and 2) when analyzing edge cases and debugging, I was starting to break as much as I was fixing. So I decided I'll start from scratch, with TDD this time.

I get the TDD cycle (red, green, refractor). The question is, -what- does one test? For an individual unit/function, how many tests do you write? In this case, I already have the whole project in my head, even though there will be a few library, and structural, changes between the languages.

Also, I've heard ad nauseam to use asserts everywhere, so I am. But sometimes it seems I'm writing tests to verify my asserts. And it seems like a waste of time to write a valid input type test for every argument, one function at a time.