In Dragon Ball, Piccolo came to life when his father sharing the same name, spat him out as an egg. His egg was first discovered by a family in the wilderness. Once the egg hatched, kid Piccolo emerged from the egg and started to destroy things.
A few years later, Piccolo was grown up and ended up competing in the world martial arts tournament where he planned to kill Goku. During the Preliminary rounds of the tournament, Piccolo fought Chiaotzu and killed him.
When Piccolo met up with Goku in the finals, the two fought fiercely. The fight started out somewhat normal but turned into an awesome spectacle of power. Piccolo grew into a giant hoping to defeat Goku, but he would eventually be thrown to the ground by Goku and returned to normal form. Attendees of the tournament fled from the fight in fear of their lives as Piccolo grew even more hostile. Piccolo destroyed the whole stage and
surrounding area when he fired a huge energy attack at Goku. When Piccolo thought he defeated Goku, he was distracted by Master Roshi, Krillin, Bulma, and Chi Chi. While Piccolo was being distracted, Goku used a Kamahama wave to gain a lot of speed and he came crashing head first into Piccolo's stomach, causing Piccolo to fall down in pain.
After the fight, Korin came to give Goku a Senzu Bean to heal and restore his energy. Goku ate one and gave a second one to Piccolo, hoping he would change his ways. Unfortunately that was not the case; Piccolo got up and told Goku he would someday kill him as he flew away. Everyone was shocked by Goku's decision and hoped it would not someday come back to haunt them.
When Piccolo was wished back to life on Planet Namek he quickly senses Frieza's energy and starts to travel towards it. On the way he comes across one of the last Namekians, Nail. Piccolo is informed that he is no match for Frieza but if the two fuse they just might stand a chance. Piccolo is hesitant but finally agrees. They decide on keeping Piccolo's body and when the fusion is over Piccolo goes on and on about how great he feels.
Piccolo and Kami also fused later on in the series in an attempt to defeat Cell. Since they were once a single being, rejoining the two halves made them much stronger than if one of them fused with someone else. Before the 2 fused they decided on keeping Piccolo's body since it was much stronger & younger. This fusion resulted in a powerful Super Namek and was the strongest being on the planet at the time of the fusion.