What resource do you recommend to start with Test Driven Development?

I've never been able to get my head round the concept of Test Driven Development. Maybe it's because, every time I've seen an example of it [which, like the one you linked to] are usually aimed at explaining the concept, they seem like a lot of extra work for questionable benefit.

For example: I've got some 'multiplying' code that should return 4 when passed 2 and 2 so I'm advised to write an "assert" test that returns 4 when I send it 2 and 2. But what happens if I send it just 2?... or 2 and 2 and 2?... or 2 and 3?... or 2 and W? None of the examples of why it's so vital to write tests ever seem to cover this kind of thing? They just seem to show a test which returns the correct answer when given the correct data. But aren't many of the problems with software caused when a piece of code receives the wrong data in the first place?

Genuine question. I've read so many times about why I should practice TDD that I believe it in theory. But I've never seen a beginner's example that has actually seemed like it would provide the claimed benefits. Why is writing a test that takes one single specified input and checks for one single specified output "better" than just writing my code and then trying to break it by [metaphorically] hitting the "Do Not Press!" button, by throwing as much incorrect, malformed or otherwise "wrong" data at it, as possible.

TDD by Jason Gorman is a nice (and free) introduction: http://codemanship.co.uk/tdd_jasongorman_codemanship.pdf (disclosure: I did Jason's ~TDD training course a number of years ago and gained a lot from it - but the book is useful on its own).

A book that explains TDD in the context of a much larger example application is Growing Object-Oriented Software, Guided by Tests by Steve Freeman, Nat Pryce (sometimes referred to as "The GOOS Book").

One of the difficult things about TDD is that it takes a long time to learn to do it effectively. I personally didn't really "get" TDD until I paired with a few much more experienced developers, and this forced me to rethink the way that I approached writing software. Ultimately, it caused me to significantly level up my coding skills and I wouldn't go back to the way that I used to write code before.

I’d recommend you stop, but if you’re really going to go through with it. One of the best resources is the testing goat book. It’s tool specific (django/python) but it’s so good I think it’s worth it just to get into the subject.

https://www.obeythetestinggoat.com/pages/book.html#toc

I would highly recommend reading Kent Beck's book. He walks you through the entire process. He has two examples, one in Java and one in Python. Both are readable even if you do not program in either language.

You could try https://github.com/bitfield/ftl-fundamentals

None! Don't do it! It's a foolish and dogmatic way to make software, and a waste of time.