Personally, I would try one of the really gooduns, then when you have decided you like them, go back and start at the beginning and work through. The timeline of the storys goes along with the order the books were released, so as far as finding out which one goes next, just look at the bit where it says "the 5th discworld book" or whatever.
As for which one to read first; I would read Mort first. From past experience I think its the one with the most appeal to people who are not already fans. Hell, my mum hates the other books, but loved Mort.
Alternatively, get one with Death in
But once you have read that, definately read them in chronological order. Theres one hell of a lot of in-jokes in the discworld series that you just wont get if you read them out of sequence. You wont even notice that there is a joke there to miss.
Plus, theres a little bit of a lull in the middle. The first books are really funny, then theres a brief period of slightly rubbish books, then they have great stories, but are not that funny. His last few books have had both the great stories
and been really funny, which is a relief.
As to the games: Ive played all of them and I can honestly say that pretty much the
only jokes in there are in-jokes, and they are not very funny ones. Didnt like em at all, and this is coming from a major discworld fan.
Didnt like the animated series either. Just wasnt funny.