You might want to read the first book of the series (The Colour of Magic) to familiarize yourself with the universe of Discworld and the City of Ankh-Morpork. However, the first book was average, so you might want to search the Web for info instead. This is enough:
http://en.wikipedia.org/wiki/Discworld_(world)The books have different protagonists. With the exception of the first two books, you can read the other books regardless of volume number. However, some of them feature the same protagonist, so it is better if you read the first book where that protagonist appears (example: read Going Postal first before Making Money. Both books are about Moist von Lipwig) This has the list of books in chronological order with protagonists:
http://en.wikipedia.org/wiki/Discworld I'd recommend reading the books that feature Death because he's hilarious but insightful in his own way. Here's a list:
Mort (highly recommended)
Reaper Man
Soul Music
Hogfather (highly recommended)
Thief of Time
British humor rules. Have fun!