Good Omens (written with Neil Gaiman) is a lot different from the Discworld series. It was okay, but not nearly as good as even a mediocre Discworld book.
Also stay away from Strata, which is a book about a Discworld but not THE Discworld.
I think Guards, Guards is the consensus choice of a place to start, but I started randomly with The Last Continent, and loved it, though looking back it was a miserable first encounter with Pratchett.