Let's dive right into the stories reprinted here!
Superman...He's The Key, The Granddaddy Of Them AllAction Comics #1 (June, 1938) by Jerry Siegel and Joe Shuster, and the one page origin they provided was reprinted here.
It had the young Superman launched from his exploding planet, to end up in an orphanage, where he exhibited extraordinary strength. Later, he also had superb speed and incredible endurance, all of which allowed him to take up his career as Superman.
A pretty bare bones origin, to say the least, but more would be added to the legend of Superman and his history.
Probably the best of all Superman origins, this was expanded upon to make the story in Action Comics #500 (and even included a mention of Superboy's history with the Legion of the Super-Heroes!).
Batman...He Was Unable Not To Fight CriminalsDetective Comics #27, but not getting an origin until Detective Comics #33, with the 2 page version reprinted here the slightly altered version from Batman #1 (Spring, 1940, which also saw the first appearances of Joker and the Catwoman) with words by Bill Finger, art by Bob Kane (with backgrounds by Sheldon Moldoff), with the story of how a mugger shot and killed Bruce Wayne's father and mother, Thomas and Martha, with young Bruce dedicating his life to strike fear into criminals' hearts as the Batman.
A simple enough origin, and the basis for every origin that followed.
Both of these origins did fit under the umbrella of the Golden Age Batman, but a much longer origin for Batman (as well as his friends) did surface later, in the Untold Legend of the Batman, which added many details added to Batman's origin from the Silver Age (like his father being a Batman, as well as Bruce's stint as Robin).
Wonder Woman...The Female Sex Is Assuming Day By Day A More Dominant Role In World Affairs
In Wonder Woman #1 (Summer, 1942, the tale reprinted here), the story of how Hippolyta carved young Diana from clay, raised her on Paradise Island, saved pilot Steve Trevor, and competed to go back with Trevor to the mainland to fight in World War II (and taking on the identity of military nurse, Diana Prince) was summarized by William Moulton Marston (under the pen name of Charles Moulton) with art by Harry G. Peter.
This would be the definite version of Wonder Woman's Golden Age origin, and the one that formed the basis for the many that followed.
Sadly, this little era of Wonder Woman, fitting between her time as a powerless Wonder Woman and the twelve trials of Wonder Woman to rejoin the JLA (including one issue of Supergirl with Hippolyta and the Amazons) has not been reprinted elsewhere.
The Flash...You Can Do Almost Anything With Him...
Eventually adopting the identity of the Flash, he saved Joan from the Faultless Four, eventually helping to found the Justice Society of America, and being there when it ended in the 1950s (and having many stories)....then getting revived with the help of the next young man....
Sometime after this, Barry would help found the Justice League of America, and later, finding the original Flash, Jay Garrick, on an alternate Earth (which would eventually lead to Earth-1/Earth-2, with Barry on 1, Jay on 2), and later, so many other Earths, as well as a Crisis, but it all started with Barry and Jay, and the Flashes of Two Earths.
Green Lantern...No Evil Shall Escape My Sightmany solo cases, facing foes like the Gambler, Vandal Savage and the Icicle.
Green Lantern was a part of the team when it ended the first time, and being there for its first revival.
Hal also helps to found the Justice League of America, finds out about the Green Lantern Corps (and that he is the protector of Sector 2814), and works for the Guardians of the Universe (who know a little bit more about the beginnings of evil, as well how the magic that formed Alan Scott's ring, than they let on).
The Green Lantern Corps are the successors of the Manhunters as well.
Hawkman...The Weirdness Seems To Be Part Of Hawkman's Appeal
Later, Shiera becomes Hawkgirl and assists Hawkman with his missions. Hawkman is also a founder of the JSA, and the only member to be involved in every JSA case in the Golden Age.
Using Thanagarian science (including the Absorbascon, which allowed them access to all of Earth's knowledge), ancient weapons and an ability to speak to birds, the duo took up crimefighting as Hawkman and Hawkgirl, and Earth alter egos as Midway City Museum curators, Carter and Shiera Hall. Hawkman joined the Justice League soon after its founding, with Hawkgirl joining the JLA later. Unlike Flash, Green Lantern and Atom, the Hawks of two Earths never worked together in the Hawkman title, though the Hawkman of Earth-1 regularly worked with the Atom of Earth-1.
Hawkgirl later took on the name Hawkwoman, then, after the Crisis on Infinite Earths and Invasion!, Hawk history got confusing (as this history of Hawkgirls will show).
Green Arrow...A Streamlined Robin Hood
This Green Arrow never joined the JSA, but instead joined the Seven Soldiers of Victory, and had many adventures with them, as well as only with Speedy.
Green Arrow illustrated by Jack Kirby. Speedy then got a slightly newer origin as well, though not until Kirby had left Green Arrow.
Green Arrow was the first non-charter member of the Justice League of America, and later became involved with the Black Canary after getting a new costume working with Batman and losing his fortune.
Introducing The Mighty Atom
Atom is a founding member of the Justice Society of America, and, later, gets actual super-strength as well as a costume change among his many solo adventures.
Atom joins the Justice League after Green Arrow, and faces many foes, including Chronos. Jean and Ray even marry later on, after Ray reveals his identity to her, but they later divorce.
Shazam!...In One Thing A Sense Of Fun And The Whole Sense Of Adventure
Billy eventually gathered a whole Marvel Family around himself, including his sister, Mary, and their friend, Freddy Freeman, and later, Tawky Tawny and Kid Eternity. Thrown into suspended animation, they revived in the 1970s, and had more adventures on their own Earth, that of Earth-S, until the Crisis on Infinite Earths.
Plastic Man...One Of The Wildest Imaginations...
Later, Plastic Man started working with Woozy Winks (a shifty indiviual himself) and the two stopped odd criminals for as long as Quality Comics lasted. Later, Plas and Woozy found themselves at home at DC Comics, with a DC Special, reprinting some of their earliest work together. The original Plastic Man later became a member of the All-Star Squadron, and, long after the Crisis On Infinite Earths, Plas joined the JLA as well (at a time when there was only one Earth).