Where's a good place to start? Is there some overarching narrative or can you read them in any order?

While there are myriad reading orders that have been devised over the years, my method with long extended series such as this one is simple: I consume them in the order in which they were produced/released.

I also haven't read it, but I've seen this graphic in the past and been fascinated by it without having read any.

I personally love the city watch books the most, which starts with Guards! Guards! and circle around themes of justice and social inequality in a way that will be satisfying if you are working class left of center. There's a sort of a timeline, but I read them wildly out of order and didn't feel the worse off for it. Pratchett really found his voice after writing a few books, so I think it's generally suggested not to start at the beginning. Each 'series' has a core motif it pokes at, but if you were only to read one to see if you liked the style... I think Going Postal?

I started with Small Gods. Which is a standalone story that takes place early chronologically. After reading that as a test to see how well I would like it. I then went to the beginning in publishing order.

Part of what makes the books brilliant is that, while there is connectivity between some of them, they are all self contained. Pratchett managed to do this without being repetitive or assuming that the reader has read all the books before the one being read. There are times when characters refer to previous books but it never happens in a way that makes you feel like you are missing something. It is useful to read the different series in order but not necessary.

Thanks for all the replies everyone!

It's non-linear to me but if something is mentioned in the book that happened in the past, I just read that book in the frame of being a prequel. It hasn't really turned into anything spoiler heavy. Though the City Watch novels are a lot more linear and tend to see a lot of character growth.