All good suggestions and in some ways it doesn't really matter as PTerry's books are all stand alone in a way - but with the 'serial' books probably best to start with the 'lead-ins'.
Starting at the very beginning works but on several re-readings now I find the 1st three (Colour of Magic; Light Fantastic & Equal Rites)
very different in tone to the rest of the Discworld genre - slightly darker and not quite so 'generously' satirical...? Sourcery
onwards they kind of start to get into gear with the humour and intelligence we all love.
12 year olds being generally quite sophisticated these days perhaps it's best to go with her preferences and -
if she likes Witches
then go with Equal Rites
or Wyrd Sisters
(on the basis that WS is actually a more proper introduction as it's the first of the coven series proper)
- Guards! Guards!
The Time Monks
- Small Gods
- but I would say that wouldn't I!
Nice little doggies...?
- Moving Pictures
(to please Chris
) - Carpe Jugulum
Yikes - use a pin or do eeny miney?