Quite simply, Terry Pratchett is the funniest man alive. However, his early books are somewhat coarse, and the later books may confused a newcomer.
Small Gods is an excellent starting point for anyone. The book has new characters, a new plot, and nothing is expected of the reader. Its a wonderful book that will explain everything for someone who's never ventured into the Discworld before. Its also one of the funniest texts around.
Small Gods is also a great books in its seriousness. The book takes a witty look at the perils of making religion too organized - in worshipping the Church rather than the God. It is a book you can read, then mull over for hours - if you didn't break up laughing every two minutes.