Pratchett books can generally stand on their own without confusing you too much with back story, so it's really just a matter of what you can find and what type of thing you like.
These books stand alone almost entirely:
Pyramids
Moving Pictures
Small Gods
The Truth*
Going Postal*
There are several series of books that share most of the same characters, so if you want to break it up into smaller chunks that's a good way to go:
The "Rincewind" series - centers around a Wizard who can't really cast spells but is really good at running away.
The Colour of Magic
The Light Fantastic
Sourcery
Eric
Interesting Times
The Last Continent
The Last Hero
The witch series - centers around witches in a small mountainous area, and features some of the best characters ever in the history of best.
Equal Rites (More about Wizards, but the first appearances of the witches)
Wyrd Sisters
Witches Abroad
Lords and Ladies
Maskerade
Carpe Jugulum
The death series - My personal favorite, centered on Death and how he learns about and is influenced by humanity. Soul Music contains much that is awesome about music.
Mort
Reaper Man
Soul Music
Hogfather
Thief Of Time
The Watch series - Follows the Ankh-Morpork Night Watch as it develops from useless into an actual force for good. Cop drama.
Guards! Guards!
Men At Arms
Feet Of Clay
Jingo
The Fifth Elephant
The Truth*
Night Watch
Going Postal*
(*Though these books don't feature the Watch as main characters, they take place in a very established city after other world-changing events, and should probably be read after the Watch series to avoid confusion)
So, yeah. They're addictive. It's fantasy from a sci-fi mind.