Just to let you know, Pratchett's writing improves massively over the series. This means, if you read it in chronological order, you start with the least good books (Colour of magic, and light fantastic).

There are multiple story arcs, however, that can be read semi-independently. The "Witches" arc, or "City guards" arc are an easier in point for many people.

I personally read them in chronological order. I knew the earliest books wouldn't be as good, so got to watch his writing improve, and the world crystallise. I knew about the slow start however, and so wasn't put off by the "average" level writing, at the start.