Answer by taking a quote from the following article:

For the upcoming film about the superhero character, see Aquaman (film). Aquaman is a fictional superhero appearing in American comic books published by DC Comics. Created by Paul Norris and Mort Weisinger, the character debuted in More Fun Comics #73 (November 1941). Initially a backup feature in DC's anthology titles, Aquaman later starred in several volumes of a solo comic book series.

Aquaman's adventures continued to be published in Adventure Comics through the 1940s and 1950s, as one of the few superheroes to last through the 1950s in continuous publication. Starting in the late 1950s, new elements to Aquaman's backstory were introduced, with various new supporting characters added and several adjustments made to the character, his origins, his powers, and persona. The first of these elements was the story "Aquaman's Undersea Partner" in Adventure Comics #229 (October 1956), where his octopus sidekick, Topo, was first introduced. This and subsequent elements were later, after the establishment of DC's multiverse in the 1960s, attributed to the Aquaman of Earth-One.  In Adventure Comics #260 (May 1959) and subsequent Silver Age comics, it was revealed that Aquaman was Arthur Curry, the son of Tom Curry, a lighthouse keeper, and Atlanna, a water-breathing outcast from the lost, underwater city of Atlantis. Due to his heritage, Aquaman discovers as a youth that he possesses various superhuman abilities, including the powers of surviving underwater, communication with sea life, and tremendous swimming prowess. Eventually, Arthur decided to use his talents to become the defender of the Earth's oceans. It was later revealed that he had, in his youth, adventured as Aquaboy and on one occasion, met Superboy, Earth's only other publicly active superpowered hero at the time. When Arthur grew up, he called himself "Aquaman".  It was later revealed that after Atlanna's death, Tom Curry met and married an ordinary human woman and had a son named Orm Curry, Aquaman's half-brother. Orm grew up as a troubled youth in the shadow of his brother, who constantly bailed him out of trouble with the law. He grew to hate Aquaman not only for the powers that he could never possess but also because he believed that their father would always favor Aquaman. Orm disappeared after becoming an amnesiac and would resurface years later as Aquaman's nemesis, Ocean Master.  Aquaman's ability to talk with fish eventually expanded to full-fledged telepathic communication with sea creatures even from great distances. He also retroactively developed a specific weakness akin to Superman's vulnerability to kryptonite or Green Lantern's vulnerability to the color yellow: Aquaman had to come into contact with water at least once per hour, or he would die. Prior to this story, Aquaman could exist both in and out of water indefinitely.

how did he end up in Atlantis?
Arthur decided to use his talents to become the defender of the Earth's oceans.