All Saiyans are naturally evil. However, Goku is an anomaly because he was dropped on his head as a child, and was good from then on. He was also the first Saiyan to turn Super Saiyan in over 1000 years. Goku is also the only pure Saiyan to turn Super Saiyan 3 (Gotenks also turns Super Saiyan 3, but is the result of two Halfbreed Saiyans fused).
Goku's childhood is detailed in the original Dragon Ball series.
Son Goku is believed to be partly based on the Chinese folk hero monkey, Sun Wukong from the story of Journey to the West. "Son Goku" is in fact the Japanese pronunciation of the name Sun Wukong.
In the original Dragon Ball series, Goku shares many traits, skills and tools with the character Sun Wukong, obviously homage to the story Journey to the West. This includes that both characters are monkeys (saiyans turn into monkeys), and thus have tails. They also both have useful tools like a magical cloud which they can summon to ride on and a handy as-you-will resizeable cudgel.
Goku also meets other characters which was inspired by the original story, including the perverse pig (Zhu Bajie).Origins