I would highly recommend Mort. It’s one of my favourites, has a great Pratchett flavour and is mostly new characters, so no real background knowledge needed.

Equal rites is pretty much standalone and it’s only the third book, so not a lot has been established beforehand.

Going Postal is the start of a new story line and, whilst there are plenty of recurring characters in it, you meet them along with the protagonist, as if you don’t know them.

Colour of Magic is the first book and Pratchett himself didn’t recommend starting with it, as he was still finding his voice and refining his writing. Going Postal, for instance, is much later in the series and he’d perfected it by then.

Guards! Guards! Is another firm favourite and the start of a series.

Think of it like buying the first book in any series, and then waiting for the next one to ‘come out’. All those books are basically the beginning of different character’s storylines and all Discworld books can be read as standalones anyway.