Ok, since you asked.
His first three novels (in chronological order) are just average fantasy. (The Colour of Magic, The Light Fantastic, Equal Rites) I recommend instead the movie "The Colour of Magic" which includes the first two books, was quite well done, and is, frankly, better than the books.
I recommend that newcomers start with Mort, (the fourth book to be published) which is a classic "Death takes a holiday" story and gives you a glimmer of the amazing author Pratchett would become. Then read Reaper Man, Soul Music, and Hogfather, which continues the story started in Mort. (Hogfather had a decent film adaptation in 2006.)
Then, go back and read Wyrd Sisters, Witches Abroad, Lords and Ladies, and Maskerade.
Then go back and read the Guards novels, starting with "Guards! Guards!", Men at Arms, Feet of Clay, Jingo, and The Fifth Elephant. All of these are good, but the real point of reading them is that they lead up to Night Watch, which is in my opinion where Pratchett crosses the line from "fantasy" into Literature. (Thief of Time is arguably a prelude to Night Watch and should probably be read first.)
After Night Watch comes Thud, which is another truly exceptional novel.
There is also a thread of novels involving the Rincewind character from The Colour of Magic, some of which are very good. (Check the wiki for the reading order.)
There is also the Tiffany Aching series, which is intended for a younger audience but is still an excellent read.
There are several stand-alone novels, Pyramids, Small Gods, Moving Pictures, that take place on the Discworld and may include characters from other books. They are well worth reading.
Pratchett has had a few duds. Monstrous Regiment and Unseen Academicals weren't very good. (I mean, I understand what Pratchett was trying to do in each case, but they just weren't as funny or clever as the others.)
But the rest ... you are in for a long stretch of being amazed, amused, and truly moved.
Be sure to read the footnotes! Some of the best stuff is there.