The Colour of Magic
The story starts in Anhk-Morpock, the most ancient city of the discworld, with the visit of its first tourist, Twoflower. Twoflower has come from a much richer part of the world, and wants to find out about some of the figures of legend., but seems to cause chaos whereever he goes. Rincewind, not much of a wizard, but quite used to dealing with trouble, decides he has to protect him, and the pair are soon spending their time keeping one step ahead of disaster - which they always manage to do, maybe because they are key characters in a game played by the Small Gods of the discworld.
A work like Tolkien's Lord of the Rings has great depth, but is also very long and difficult to read all the way through. Pratchett does things differently, developing his world through a large number of books. Having read the first, I didn't feel that I simply had to mopve on to rest of the series, but I probably will The Light Fantastic sooner or later, as the books are easy to read and seem just right for times when you need a bit of amusement.