This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
The first story he ever wrote was for a school assignment and was called
"The Hades Business". He managed to get 10 out of 10 for it. It was the
first time he had gotten 10 out of 10 for anything except for a painting
which his teacher *thought* was two dinosaurs fighting. This story was
then published in his school magazine.
There it would have ended, except for his school headmaster who
addressed an assembly shortly afterward and announced that he didn't
approve of the "moral tone" of the story. The magazine, which would
have struggled to break even, sold out within 15 minutes. He learned
an important lesson, right then -- by writing it is possible to
infuriate your enemies as well as please your friends.
He then had the story typed up by his Aunt and sold it to the magazine
Science Fantasy (#60, vol 20, 1963), and with the profits bought a
typewriter. This his first act as an income earner was to fire his Aunt.
His mother rewarded this Thatcherite attitude and paid for his typing
lessons and he was on his way.