Starting with Brave and the Bold #28 (February-March, 1960), and facing off against "Starro the Conqueror"...so came together the Justice League of America by writer Gardner Fox and artist Mike Sekowsky. The team continued along with two more tryout issues (Brave and the Bold #29, facing Xotar the Weapons-Master and Brave and the Bold #30, fighting Professor Ivo and his adapting android, Amazo), before getting a Justice League of America #1 (October-November, 1960), in a title that ran 261 issues (and 3 annuals), ending its original run with the cover date of April, 1987.
Here's a little on the original seven founding members of the Justice League of America and where they came from and who they were before they joined the Justice League!
Who to start with? How about....
The Man of Steel
The Superman who was in the Justice League also had a career as Superboy (which the original Superman, Kal-L, didn't have), so technically More Fun Comics #101 (January-February, 1945, also by Siegel and Shuster, and no cover appearance until #104) could be considered his first Earth-1 appearance...and Superboy's first mention in a Superman comic was in Superman #46 (May-June, 1947), so that could be considered Superman's first Earth-1 story!
The Darknight Detective
Batman began his unrelenting war against crime...though that wasn't the Batman that joined the Justice League (nothing nefarious, just a change in...well, the universe).
first meeting" with Superman in Superman #76 (May-June, 1952), and oddly enough...that ends up being Batman's first Silver Age (or Earth-1) appearance (due to the fact that Superman and Batman met in All-Star Comics #7, both as honorary members of the Justice Society!). Batman himself hadn't changed as much from his Golden Age to the Silver Age, but boy did the world around him change!
Villains like the Joker, Two-Face, Scarecrow, and Penguin got more complex, Robin was still the Boy Wonder (at least for a time), and with Detective Comics #327, Batman added the yellow circle around his chest emblem (though he had no emblem in Justice League of America #1-27, he started sporting it with Justice League of America #28).
Manhunter from Mars
This also cemented J'onn's relationship with the Justice League, and he's been a mainstay there for decades.
legacy (Barry was reading a comic of the earlier Flash's exploits! Barry was the beginning of revamped heroes, with more scientific origins, and the start of a whole new frontier of super-heroics).
the Rogues (Captain Cold, Mirror Master, Pied Piper, Weather Wizard and Captain Boomerang popped up in JLA), as well as villains like the always changing Dr. Alchemy (who was also Mr. Element), the future magician, Abra Kadabra, and the villainous speedster, the Reverse-Flash! Barry also met that other Flash as he traversed worlds, and started a tradition, wherein the Justice League met with the Justice Society, and the two teams dealt with whatever Crisis would come up...and try to defeat it together!
raced against Superman, had a sidekick in Kid Flash (aka Wally West), made friends with the Elongated Man (also a future JLAer), married his fiancé, Iris West, discovered a city of talking apes in Africa (and faced Grodd, a gorilla with mind powers, to help the leader of Gorilla City, Solovar), and even got to meet his maker, editor Julie Schwartz! Due to some personal hardships, Barry did have to take a leave from the Justice League towards the end of its run....but Barry was always there for the JLA, and they for him. All this coming from a man who just wanted to good for everyone around him...and did it very fast, thanks to a bath in chemicals electrified from a lightning bolt!
hard time travelling the country with Green Arrow, Hal came to find himself again (but he did so as a back-up feature in his buddy's, the Flash, title, as his title had been cancelled for a time), and Green Lantern foes like the immortal mind reader Hector Hammond, the sound weapon using Sonar, master of living body art the Tattooed Man, magnetic manipulator Dr. Polaris and even the hyper-evolved Shark all ended up facing the JLA for a time (while yellow ring-slinger Sinestro only menaced Hal with the Secret Society of Super-Villains, along with a new girl in the guise of Star Sapphire).
Hal even had a few back-up Green Lanterns, with John Stewart ending up working with the JLA briefly during its original run (and Guy Gardner being a mainstay after the Justice League's rebirth). Hal did get his own title back, and with his title recharged still hung around with the JLA (as well as going back to work for Carol Ferris at Ferris Air)...at least until Aquaman reorganized the JLA after the "Martian" invasion, and took the team to Detroit.
Black Canary, the Elongated Man, the Red Tornado, Hawkgirl, Zatanna and more..
...but that is a story for another day!