What was the first character Stan lee created?

Stan Lee's first superhero co-creation was the Destroyer, in Mystic Comics #6 (August 1941). He was 19 at the time.

He continued to create many characters over the years, but it was his co-creation with Jack Kirby of the Fantastic Four (November 1961) that started the "Marvel Age of Comics" and ushered in other heroes like Spider-Man, Hulk, Thor, Iron Man, Silver Surfer, X-Men, etc.