Reverend has it pretty much right. You can start with any of the first of the subseries, except for Equal Rites, start with Wyrd Sisters instead. Wyrd Sisters-Witches Abroad-Lords and Ladies makes a pretty good trilogy for example.
I came to the DIscworld because of SFX Magazine. I'd seen the books at the library and been put off by the covers, but SFX gave away a book with the first chapter of a number of different books, among which was the opening for Mort. I nearly injured myself laughing, and rushed out and got as many of them as I could, and asked the library to get more.
Once you've got a few under your belt,
then you could read the Rincewind books, Colour, Light, Sourcery, Eric are all OK, but for me he really comes into his own with Interesting Times, which also includes the mighty Cohen the Barbarian and his Silver Horde (one of my favourites).
The interesting thing about the whole series is the longer it goes, the deeper they get. They aren't quite as gut bustingly funny, but wow, the characters do things that have you going, "Hmm, didn't see that coming".
And don't underestimate the Tiffany Aching novels. I'll nail my colours to the mast and say they are hands-down better than Harry Potter! Yeah, I said it!
I go also with the opinion that the City Watch stories are the best, and among most fans Night Watch is rated as the best DW novel of all. But as a character Granny Weatherwax is in a league of her own, and in the Witches books, Lords and Ladies is another favourite.
As for the standalones, they aren't a bad place to start, especially Moving Pictures (if you know anything about movies) or Pyramids (if you know anything of Egyptology).
And in conclusion, when I ask fans what's their personal favourite, not necessarily the one they think is the best, that one is also usually the first one they read. For me, not knowing the sequences at the time, that was Men At Arms. I still rate it as my personal favourite, because it introduced me to that wonderful, weird world.
Oh! BTW, there are various DW TV things out there. They aren't bad, but not a patch on the novels, except perhaps for Going Postal. I would recommend the soundtrack for the cartoon version of Soul Music, there are some pretty good songs on it, like 'Pathway to Paradise', listen to it when reading the book.
