this post, and recently Debra reviewed his latest (the 39th in the series), Snuff. If you read those posts you know already that you don't have to start reading the series with book #1. In fact, you can start pretty much anywhere. Everything you need to know is contained in each book. And even when information is repeated, Pratchett is clever enough to keep it interesting and funny every time.
That said, there's also no reason NOT to begin at the beginning. The first book in the series, The Colour of Magic, introduces readers at a somewhat greater length than the others to the cosmology of Discworld: Discworld is not a globe like Earth. Instead, it is a disc, this disc is help up on the backs of for enormous elephants. In turn, the elephants are standing on the back of a giant turtle, known as the Great A'Tuin. The Great A'Tuin, thus loaded, swims through space.
On the surface of Discworld, things look a little bit like they do on Earth, except with a lot more magic, magical creatures (witches, wizards, trolls, goblins, elves, dwarves, vampires, werewolves, Death, gods, demons, etc.), and a lot less of modern technology. In The Colour of Magic, a tourist (apparently the first that Discworld has ever seen) visits the seedy city of Ankh-Morpork snapping pictures with his demon-powered "picture box" and paying merchants large gold coins which he stores in self-propelled, semi-intelligent Luggage. The tourist, who goes by the name of Twoflowers, chooses Rincewind, a luckless dropout from wizarding school, as his guide. Twoflowers is determined to see every bit of the "quaint" and "picturesque" Ankh-Morpork despite the fact that most of the city--including, perhaps, his own guide--is out to rob and/or murder him.
If you have read other books in the series, you'll find The Colour of Magic interesting in what it reveals about the creation of Discworld. Many of the recurring characters and themes that define the series make their first appearance here. Death is about, regularly feeling cheated. The City Watch takes part, though the individual characters that make it up have yet to be revealed. Through Rincewind we learn much about Unseen University.
Like the other books in the series, The Colour of Magic is populated with deeply flawed characters, people who lie or cheat or are so naïve they deserve to be cheated but who are, nonetheless, entirely recognizable and entirely loveable.
There's nothing at all wrong with beginning at the beginning.