Clark Kent learned the details of his home planet Krypton very slowly, as there are few sources of information for him. When Clark was 14, his adoptive father Jonathan Kent told him that he came from another planet; Clark didn't know which one or anything about it.
Nearly two years later, Dr. Virgil Swann told Clark that the name of his home planet was Krypton. Dr. Swann also told him that Krypton was destroyed, but didn't know how Krypton was destroyed, but guessed it was because of war, famine, or disease. When Clark returned home from visiting Dr. Swann, he activated his spaceship. Inside, he found a message from his Kryptonian father Jor-El, which read: "On this third planet from the star Sol, you will be a god among men. They are a flawed race. Rule them with strength, my son. That is where your greatness lies." Clark was horrified to think that he was sent to conquer Earth, leaving him to conclude that Krypton was a planet of warriors.
Just after Clark's 16th birthday, the artificial entity programmed with the consciousness of Jor-El began communicating with Clark, and instructed him to leave Smallville to begin preparing to fulfill his destiny. When Clark refused, Jor-El burned a Kryptonian symbol on his chest, leading Clark to the conclusion that Kryptonians were cruel. Later, when Clark found a memory pendant which had belonged to Jor-El decades earlier, he learned that Jor-El's own father had been harsh with him as well.
Nearly a year later, when Dr. Lawrence Garner placed him in a memory well filled with kryptonite water, Clark remembered his last moments on Krypton, which he remembered as a colorful place. He also realized that his Kryptonian mother, Lara, was not cruel or harsh; she loved him very much. When he was first placed in his spaceship, Clark was only a few months old, so his journey to Earth must have taken approximately 2 to 3 years. Additionally, the Kents produced adoption paperwork that indicated Clark was about two and a half when they found him.
There were teleportation portals leading from Krypton to Earth, but were sealed by the Kryptonian Science Council so that General Zod and his followers could not escape, forcing Kara and Kal-El to get off the planet by taking spaceships along a three-year journey through space to reach Earth.
When in the Fortress of Solitude, Clark heard transmissions from Kara on the planet in the year 1986. Later, after returning from an alternate reality Jor-El showed him, Clark used a time portal in the Fortress to return to Krypton before it was destroyed and got there just as Brainiac was about to kill baby Kal-El. Before he could, Brainiac battled Clark and Kara, who were defenseless due to Krypton's red sun until Kara stabbed Brainiac with a shard of crystal, destroying Brainiac. Clark then placed baby Kal-El in his ship, and it flew away while Clark and Kara used the portal to get to back to Earth.
Clark's training in the Fortress has allowed him to study Kryptonian history.