Good to know...
I'd recommend the books Small Gods, and Good Omens to any Terry Pratchett starter. They work well without any foreknowledge of Discworld.
After reading those two (or one of them) I'd recommend reading all the other books he's ever written...but that's just me...
The nice thing about Terry Pratchett is that he wrote a huge number of good books and they all belong in the same "Series" but are still all seperate novels that can be read standalone. You might miss some reference and not get osme of the jokes, but you'll still be able to enjoy every novel on it's own. One of my "Plans for Life" is to read all Discworld novels in their proper order...so far I just grabbed any book and read it...which is also fun...
My favourite characters are:
Rincewind (especially in The Last Continent)
The Luggage
DEATH
Susan
But I really like all the other characters as well...
The witches are actually my least-favourite , but I still enjoyed every book that featured them...