Ooooh, that old chestnut. There are plenty of reading order guides around, so you might find one just to make sure you keep track of all of them.
What I did was just start from the first one (Light Fantastic & Colour of Magic) because I am a stubborn little shit. Reading in ordinary chronological order will give you the fullest experience and make you really appreciate the slowly evolving worldbuilding, plus there is a lot of crossover bonuses you lose when you don’t read chronologically. So if you’re a quick reader and you’ve got a lot of time on your hands, do that.
Otoh, the reason why people often don’t recommend starting with The Colour of Magic is that it isn’t really typical of what Discworld really is. It’s still very pastiche, parody of traditional fantasy, with a lack of the character depth and realism that makes the rest of Discworld so satisfying to read. Mind you, they’re still good, but they’re not representative.
So, alternatives: there’s a couple of standalones that work pretty alright on their own:
- Small Gods: set some undefined time before the rest of the series, about organised religion and tyranny and Ancient Greece. You will never look at turtles in quite the same way again. Also notable for having no romantic subplot and a character that’s both morally pure and really likeable.
- The Truth/Going Postal: the way in if you’re into steampunk. The Truth takes the semi-medieval city of Ankh-Morpork and introduces newspapers; in Going Postal the industrial revolution is already well on its way and the city’s boss gets in a reformed crook to revamp the postal service.
- Monstrous Regiment: FEMINISM. A girl crossdresses to join her (backward, aggressive, religiously fanatical) country’s army to find her brother. Shenanigans ensue. Contains the closest thing DW has to a lesbian couple, some really really interesting musings about gender, and a coffee-addicted vampire. (also contains some really dark scenes)
Then there’s the subseries:
- Guards Guards: Vimes is one of the best-loved characters of the series, and with reason. And he evolves: in this first one he’s still a cynical, mostly incompetent drunk, and as the series progresses he slowly gains power and confidence and it’s gorgeous to see. All Watch books are basically CSI: Ankh-Morpork and vaguely Sherlock Holmes-like. Your way in if you like crime and detective stories.
- Wyrd Sisters: the other main subseries, ie the Witches.. Women. So many women. Notable for failing the reverse Bechdel test, that’s how many women there are in. Witches are basically the Discworld’s equivalent of social workers in rural areas, with a sidedish of magic: all Witches stories follow roughly the same plotline of the three main witches having to fight off some supernatural foe. They are also explicit parodies: Wyrd Sisters does Shakespeare (Macbeth in particular), Witches Abroad is Cinderella, Lords and Ladies is every Fae myth, Maskerade the Phantom of the Opera, and Carpe Jugulum is vampires.
- The Wee Free Men: reboot of the Witches series, basically, only with a teeny-tiny witch - nine year old Tiffany Aching has all the characteristics of Pratchett’s other witches but in the form of a kid. They’re more story-driven and less parody than the Witches proper, so if you don’t like buttloads of references, this might be best.
- Hogfather: strictly speaking you should read Soul Music before this one, but Hogfather introduces one of the other most beloved protags of DW, ie Susan Sto Helit, granddaughter of Death (it’s… complicated. details are in Soul Music). A lot of fun with the classic tropes of fairy tales, plus Susan is a wonderfully sarcastic, angry, takes-no-shit genre-savvy badass.
And then, if you don’t mind missing out on references and continuity:
- Night Watch: Sixth in the Watch series, and arguably the best of the lot. Sam Vimes travels thirty years back in time, meaning there’s a lot of dramatic irony if you know the rest of the series, but you can still follow without that. It’s dark, it’s angry, it’s depressing, and burns with revolutionary spirit.
And there’s more, of course. A lot depends on your interests. Egyptian mythology? Take Pyramids. Feudal-age China? have a look at Interesting Times. Want fun with Australia? Try the Last Continent. Etc.
Personally, my recommendation would be to first read one of the later ones, that really matches your interests, just to see if you’re into it. And if you’re hooked, start at the start, because frankly I’ve never seen more beautiful and complex world building and character evolution than in Discworld.
Hope that helped a bit, anon!