As a young man, Siegel was a fan of movies, comic strips, and science fiction pulp magazines. He would start writing his own stories and characters, becoming involved in what would become known as "fandom". Siegel would publish the first SF "fanzine", Cosmic Stories at the age of 15. At the age of 16, Siegel would meet his longtime collaborator, Joe Shuster. It was around 1934 that Siegel would come up with the idea of Superman and immediately set out on a four-year quest to publish a book about this character. Interestingly enough, Siegel and Shuster originally created a character called "the Superman", who was a bald telepathic villain bent on dominating the world.
Superman was created in a period of incubation as described by Ritter in the article "Creativity: The role of unconscious processes in idea generation and idea selection". Siegel was sleeping one night, and the idea of Superman came to him and he got up and started writing everything he could remember. The article says that sociocultural theories say that having an idea is easy, but it is difficult to develop your idea so that your audience accepts it. Siegel came up with the idea of Superman while he was sleeping in 1934 and it didn't get published until 1938. It took Siegel and Shuster 4 years to get their idea out to the people.
Superheroes remain an integral part of the comic book industry. The stories that are weaved by authors are fascinating and since then, the industry has developed to the point that superheroes are not as abundant as they were, but the fact remains that nothing we have know could exist without Siegel and Shuster's original brain-child, Superman. Siegel and Shuster both passed away in the 1990s, but I have faith there will be another creative who will dare to push the boundaries of comic books like Siegel and Shuster did.
I highly recommend watching the documentary Superheroes: A Never-Ending Battle for an interesting retrospective on the industry and the creation of iconic superheroes.
Some interesting articles about Superman himself:
https://www.youtube.com/watch?v=eTUrFYU2e_I - Part 1 in a series of interviews with Siegel and Shuster