I would go along with the above.....Guards, Guards is a brilliant read in its own right and introduces the City Watch, Sam Vimes and all the complexities of keeping the peace (!) in Ankh Morpork (AND it has dragons!!)....it's the opening to one of the richer seams of Discworld gold. Wyrd Sisters is the introduction to the witches of Lancre and their world. I would be completely at home there!
My voyage of Disc-overy was when Death was a boy and my brother handed me Colour of Magic with the words 'You might like this'. I read my way through his collection in publication order until he moved away just before Maskerade when I had to start my own collection.
Let us know how you get on VogonPoet but in prose please