Not the first time this question has been asked rosierad.
It doesn't really matter too much which book you read first as most of them read perfectly well on their own and some are 'stand alone' anyway in that they don't 'belong' with any serial book (there are 4 main 'serial's' running through Discworld). On the other hand reading them in order of publication is as good a route as any.
More on the serials -
The Wizards - start with
The Colour of Magic which is the 1st DW book anyway
The Witches They sort of start with
Equal Rites (which is also a wizard book) or 'officially' with
Wyrd Sisters
Death - He's in all of them just about but
Mort was the 1st he starred in
The Watch -
Guards! Guards!
Prepare to laugh loads - and sometimes cry too
PS edit - There are other serials but those are the 'classic' ones. The others will be in to tell you about the rest very shortly and be of more help than me 'cos I haven't read all the later serials so can't really comment well enough