Read the first 10 pages. Do Not Skip Footnotes. (He often sets up recurring jokes in footnotes, and you won't get the jokes unless you've read the footnote.) See whether you've laughed.

If I knew better what you like, I'd know which one to recommend. There are several semi-independent series and a few stand alone books. I'd suggest starting with the start of one of the series, or with a stand alone book.

Lemme look at a list. [link|http://www.fantasticfiction.co.uk/p/terry-pratchett/|http://www.fantastic.../terry-pratchett/] I'd say that good starting points include books 1, 3-14, 17, 20, and 26.

Particularly good starting points might include 13, 14, 3, 8, 12 and 6. (Yeah, yeah, 12 is in the same series as 6, but a very different setting.)

As others have mentioned, the series started with #1, The Color of Magic, but he hadn't really hit his pace yet. In particular you'll notice that a lot of characters change personalities a lot over the first few books.

My personal favorite is probably 14, Lords and Ladies. The only one I haven't read yet is the last, Thud.

If you scroll down to the children's books, I've read several of them and like them.

Incidentally in some sense the first Discworld book was Strata (which I quite liked), but that was a science fiction book and is clearly not in the same series. But it is interesting to see how ideas introduced there evolved through the series.

Cheers,
Ben