For the watch series start with guards guards, the death ones start with Mort, the witches start with equal rites, Moist van Lipwig ( industrial books) start with going postal, the Rincewind books with the colour of magic and then there are completely separate books like rolling pictures, small gods that are standalone. The Tiffany series is more young audience and starts with the wee free men. I highly recommend starting with the watch as it not only helps with world building it is a complete cast your going to love soon. The witches gave more fun with takes on shakespeare, vampire or fairy tale stories. The Rincewind books might be hardest to get into as the first two or so are a bit more complex. Same with the first of the witches being less fun and more world building. Here is a more comprehensive explanation https://bookriot.com/discworld-reading-order/ Im