This article is from the Isaac Asimov FAQ, by Edward J. Seiler email@example.com and John H. Jenkins firstname.lastname@example.org with numerous contributions by others.
When he was eleven years old he began writing
The Greenville Chums at College , which he planned to be the first book
in a series. After writing only eight chapters about the adventures of
boys living in a small town, he gave up after recognizing the fact that he
didn't know what he was talking about. However he made a very important
discovery in the process. After he wrote the first two chapters, he told
the story he had written so far to a friend at school during lunchtime.
When he stopped, his friend demanded that he continue. When Asimov
explained that he had told him all that he had so far, the friend asked to
borrow the book when he was finished reading it. Asimov was astonished to
discover that his friend thought that he was retelling a story that he
read. The implied compliment impressed him so much that, from that day
on, Asimov took himself seriously as a writer.
Asimov's first published writing was a column he did for his high school
newspaper. His first accepted piece was a humorous essay entitled "Little
Brothers", which appeared in The Boys High Recorder , his high school's
semi-annual literary publication, in 1934, and is reprinted in
Before the Golden Age . He wrote it in a creative writing class he took
that year; a class which almost convinced him to give up writing.