I second Mort, as a great starter, or Feet of Clay, or ANY of the books about the Witches... but to be fair, whichever you choose is going to have you hooked (except maybe The Colour of Magic and The Light Fantastic...I would say leave these until you're well acquainted).
Can I also say, leave 'The World of Poo' well alone. I was so disappointed. The Tiffany Aching series is ace and actually so is The Amazing Maurice and His Educated Rodents.
How bloody exciting, please come back and let us know what you chose and what you thought.