On April 18th, 1938, (cover date June, 1938), Superman and the contents of Action Comics #1 were copyrighted (with the issue likely going on sale on May 3rd, 1938). Superman, the champion of the oppressed, was released onto an unsuspecting world by Jerry Siegel and Joe Shuster with 13 pages to lead off the issue (as well as being featured on the cover)....and the world was never the same afterwards, as Superman spawned generations of other heroes and a worldwide revolution!
This story was also the first appearance of Lois Lane (based on the lady Jerry Siegel was infatuated with, Joanne, and Joanne later married Jerry!).
The issue also featured Chuck Dawson, magical hero Zatara, Sticky-Mitt Stimson, Marco Polo, Pep Morgan, Scoop Scanlon and Tex Thomson (who later fell into becoming a mystery man himself, Mr. America).
Remember, at this time, even Batman didn't exist yet (but he was coming with Detective Comics #27).