Siddhartha's friend has to go back to his home to do work, Siddhartha is curious of what he is talking about, so his friend explains to him what work has to be done and Siddhartha leaves the palace walls to go with him for the first time in his life.
On the way Siddhartha sees a dead man laying on the floor. As they walked a bit further they also as a beggar. After even a little more he sees a old man and asks what that is. His friend tells him that that will happen to him someday if he is lucky
Siddhartha Gautama sees that their might be more to life than just inside the palace. He decided to try to find the meaning of life, and becomes an ascetic priest.
He finally gives up on finding the meaning of life so on his way back to the palace he stops to rest under a tree. And that moment when he rested under the tree is where his whole mind was emptied and he finally found enlightenment and the meaning of life.
He becomes Buddha and teaches people of all social classes about a religion called Buddhism. He tells them that life is suffering, but that their is an eight-fold path that has a list of things that you can do to reach nirvana. Nirvana is the highest state that a person can be. When they have reached Nirvana they will not suffer anymore.
Siddhartha also taught the people about the 4 noble truths, which is the truth of the causes of suffering. Once a couple of people started following and learning about Buddhism, more people started to recognize it. And when more people joined more and more people discovered it.