There are several series within the Discworld series (The watch, the witches, Tiffany Aching, Moist Von Lipwig) and there are several stand-alone books as well, so .. the entry points are many.
You don't have to start with the Colour of Magic, his first book which I'll admit I consider rather weak compared to many of his others. I would suggest you start with perhaps .. Going postal, Guards! Guards!, Wyrd Sisters, The Wee Free Men ..
Books like Soul Music, Mort, Reaper Man, Pyramids, Small Gods, Monstrous Regiment and The Truth would also work as entry points, although some of these do have characters in them that appear in earlier books.