Tiffany Aching is The Wee Free Men (excellent), A Hat Full of Sky (excellent) and Wintersmith (not bad, I'd save Wintersmith until you have no other pratchett left or just have to have more Feegles in your life.). In that order.
I wouldn't bother with the non discworld novels until you've finished discworld
There's a good list here
http://wiki.lspace.org/wiki/Bibliography I'm not suggesting that some of them aren't just as good.
My personal favorites are Wyrd Sisters, Guards! Guards!, Men at Arms, Pyramids, Small Gods and Carpe Jugulum but you could grab any you like the look of. If you read The Colour of Magic, read The Light Fantastic right after since it's the only real duology (better than the movie btw). Of the discworlds, I'd save Wintersmith and Monstrous Regiment for last as I think they're the weakest but that's just my opinion.
oops, one more note. If someone is starting with The Colour of Magic, skip all the prologue, foreword, preface stuff and go straight for the story. Get into the story before you try to learn the physics and astronomy and that background stuff Mr. Pratchett thought you needed to know. It put me off from reading the book the first few times I tried and fortunately I read a later book and got hooked, then went back and realized what it was that was so off-putting. His strength isn't world painting, it's dialogue and plot where his brilliance shines. Again, in my opinion.