Pratchett is one of my all time favorites, and if I were going to list a few that are easy to get into:
Guards! Guards!
Men at Arms
Small Gods
Lords and Ladies
Interesting Times
Now, the above books are out of order (except for Guards! Guards! and Men at Arms; they are books one and two of the night watch series. Also, Small Gods is a stand-alone.), but that doesn't really matter. They are all highly entertaining on their own, and you won't be too lost.
The thing about Pratchett, though, is you have to give him 100 pages. The stories read fast, and some pick up faster than others, but after a 100 pages in, you will love the story. He does a lot of set up in the beginning, but it all pays off marvelously. Of the above books, Guards! Guards! and Small Gods are probably the two most difficult to get into, but they are also two of my all time favorites, so I feel they are worth the read. I can remember that in both books (heck, in pretty much every Pratchett book), I would reach a point where I'd have to put the book down and walk out of the room because I was laughing so hard that I was near hyperventilation.
He is very much worth your time.