The Birth: Galactus made his debut in Fantastic Four #48 while his origin was outlined in Galactus: The Origin #1.
A sole survivor of a previous universe, the mortal known as Galen was coverted by the Cosmic Egg during the birth of the nacsent reality into Galactus, a new entity of incredible cosmic powers. But because of his interaction with the Cosmic Egg, he became a part of the infant universe's very existence.

The Legend: His new life was at the sacrifice of his normal eating habits. Galactus now could only survive by consuming the eneries present on life sustaining planets.
Galactus choses a being as his herald and coverts the chosen one into a being of incredible power weilding a limited power cosmic of his own. Years ago, when Galactus arrived for lunch on a planet teeming with life, one person stood against it and aksed that his planet be spared. Galactus considered the request but asked the person to become his herald for eternity. This person became the Silver Surfer and was destined to seek out new planets with the energy that Galactus needed to survive. Much later, when Galactus arrived on Earth to dine, the Silver Surfer saw that there were intelligent living beings that would fight for their planet, even if it was a dying cause. Galactus considered humans like mere ants, not worthy of the air they breathe. The Fantastic Four fought against Galactus. And then the Silver Surfer felt compassion for the humans and tried to prevent Galactus from destroying the planet. It was only because of the Fantastic Four and the Silver Surfer that Galactus spared Earth. But he exiled the Silver Surfer to Earth for all eternity so that he may share the planet with the beings he was defending. He created an invisible barrier around the Earth space which only the Silver Surfer would not be able to penetrate.
Many times his stomach has brought him back to Earth, only to be repelled by heroes like the Fantastic Four. At one time Galactus was so depleted of energy that; while fighting the Avengers and Fantastic Four, he fell to the ground and started shrinking. The Four, along with the Avengers saved Galactus who then decided to fill his stomach. Like the Silver Surfer had done those many decades ago, Frankie Raye, Johnny Storm's girlfriend decided to become Galactus' herald in exchage for sparing Earth. Frankie was touched by the power cosmic converting her instantly into another Human Torch, only much more powerful. Before leaving, Galactus expressed gratitude to the Fantastic Four and for the first time called them friends.

Fantastic Four #49	Galactus: The Origin #1