Honestly, with the Discworld novels you can start anywhere. But I'm fairly anal-retentive when it comes to books, and I like to go in chronological order. So you really can't go wrong with
The Color of Magic and then reading
The Light Fantastic. Those are the first two, and they DO go in order. The only other Discworld novels that you really have to consider any kind of continuity are the Death Trilogy -
Mort,
Reaper Man, and
Soul Music.
I finished Theif of Time about 15 minutes ago. That was quite enjoyable, and featured Death's granddaughter, Susan. The only other book she's in (that I can remember, or as far as I know) is
Hogswatch.
So if you're anything like me when it comes to chronological order, start with the first two novels mentioned, and then work your way forward based on original publication year. My boyfriend has lately been traversing the Discworld and constantly tells me he hates me for introducing him to those novels, even as he's picking up another one to read.
As for what I'll read next...I haven't decided. Possibly another Discworld novel, Mons
terous Regiment, or I'll start reading
Deepwood by Jennifer Robertson. Those are the two most likely to be next, anyway.