Glad you Followed up the Suggestion to check out the Hogfather Movie.
And you will definitely enjoy the books even more.
In Fact I've got two of my cousins Interested in the series by showing them the Movie.
We were only going to watch the first part that night as it was getting late, but they could not wait till the next day
. In fact the next day they begged to get the books so they could get started.
As for where to start, I personally feel that the Colour of magic and the Light Fantastic is not the best books to start with, as they are nowhere near Pratchett's best work. His style is still developing, and as such does not give the best impression of the Author.
I would suggest starting with either
Guards! Guards! - The beginning of the Guards series. you don't really need a lot of background info here and is a great read. The characters and situations they are in get more complex as the series progresses, but you really need to start here to get most of the references.
Similarly, Start with Equal Rites, or better Wyrd Sisters to get into the Witches Series. While Equal rites was the first featuring the characters, Wyrd sisters was the firs where the characters resembled the characters as they are today.
Or, If You started with Hogfather, and want to know more about the characters in the movie, Start with Mort, the first in the Death Series.
L-Space has a great reading Guide, with suggestions and a lot of other stuff suc as listings of all the annotations that have been found in the books, and there are tons.
The reading Guide can be found here
http://www.lspace.org/books/reading-...des/index.html , and I've attached the Graphic version of it here for easy reference.