Trivia: Series creator John Carpenter actually co-wrote an early draft of the script and even considered directing, but his ideas were dismissed by producers as being "too cerebral" for audiences, and he subsequently dropped out of the project. His script reportedly was more focused on the psychological effect Michael Myers had on the residents of Haddonfield (and possibly inspiring other murders), with Michael only appearing in visions, dreams and possibly as a "spirit."
Trivia: Due to an impending writer's strike, the script was completed in only 11 days by writer Alan B. McElroy. McElroy cracked the story, and managed to complete a final draft in this short period of time, reportedly turning it in only a few hours before the strike began.
Answer: It's never explained who told Jamie of her murderous uncle, but Jamie is mocked and made fun of by her classmates for being related to the infamous Michael Myers. She could have been told by her adoptive family, classmates, neighbors, etc, but the movie never provides any definitive answers.