The saga of Galactus actually starts a good while before the awesome eater of worlds actually puts in an appearance in the Marvel Universe. To my mind it begins in the early issues of Fantastic Four with the debut of the mysterious Uatu the Watcher.
In Fantastic Four #13 by Stan Lee, Jack Kirby and inker Steve Ditko the Fab 4 are in a race to the Moon against the Soviet team of Ivan Kragoff (The Red Ghost) and a trio of exceedingly well-trained apes (a gorilla, a baboon, and an orangutan). Both teams get to Moon (after Kragoff and his apes get a heady dose of the same cosmic rays which birthed the Fantastic Four) and begin to battle for the weird ruins in the mysterious Blue Area territory they targeted to begin with.
Then appears an ascetic looking giant who calls himself the Watcher. He claims to be part of a highly superior race who have taken the very passive role of watching other species and races mature throughout the depths of space. Normally the Watcher declares that he keeps his distance and doesn't interfere, but he also claims that the FF and the Soviet Super Apes have invaded his home so he steps in a bit. After the battle has run its course and the Red Ghost is defeated, he says he will withdraw now that mankind has put foot on the Moon and leave us to our fate. But that turns out not to completely true.
Surprisingly after only a few appearances, the Watcher catches the Marvel Universe's attention and the bald-headed soft-spoken character even gets his own series beginning in the pages of Tales of Suspense #49. Sharing the mag with Iron Man, we get the Watcher functioning as an intergalactic Rod Serling (famous TV fourth-wall breaker) offering up morality fables from across the depths of space and time.
After that the Watcher turns up again and again, getting his first ever cover appearance on the handsome and mysterious cover of Fantastic Four #29. (One of my favorite FF covers.) This story pits the team against the Red Ghost and his Super Apes as they return to the Moon for a rematch.
The Watcher puts in a cameo at the very end of Avengers #14, in classic Twilight Zone fashion.
He shows up in the Hulk series in Tales to Astonish when Greenskin ends up inside the Watcher's distant domicile to steal an ultimate weapon for The Leader.
All of this sets the stage for his finest hour in Fantastic Four #48 when against all the protocols of his ancient race he once again caves in to his sympathy for man, and gives a warning to the Fantastic Four, and all of mankind by proxy, of the arrival of a most deadly menaces -- the star-spanning Galactus.
More on that tomorrow. And eventually we will learn that there is an even greater connection between the Watcher and Galactus than we'd have at first imagined.