Siddhartha lived in luxury in his palace. He learned about things life Hinduism and more. He never left the palace because his parents thought that the kingdom would crumble if he left.
One day Siddhartha left the palace and saw a sick man, a old man, and a dead man. So he thought for a while about the men and decided to go on a journey to find the meaning of life and suffering.
Siddhartha became an ascetic with a few other people. He ate very little and drank very little. He prayed day to night and one day a girl came and gave him a bowl of rice. While he enjoyed it he realized that he found happiness in a bowl of rice. So he set of to continue his journey a different way.
Eventually, Siddhartha got tired of his journeys after many, many days. He sat down under a fig tree. Also known as the bodhi tree. He sat there for a while and reached enlightenment. He learned about a lot of things such as the 4 truths and the 8 fold path.
He learned the meaning of life and death and all these new ideas and truths but he had no idea what to do with all this information. But while meditating Brahman, a universal soul, tells him to share his information. So he went back to the ascetics and told them what he knew.
Buddhism eventually became a world wide religion. It spread from Gupta, India, east where places like china would learn this religion. Even though Buddhism doesn't let people "join" there is many reasons why the religion spread. Like mating and reincarnation and a lot of other reasons that may be true.