When the Buddha was born, a priest came down from the mountains and told the king that there was a prophecy. Because he had a birthmark of a wheel, he would either grow up to be a great king, or a religious leader
The king had always tried to make the young Buddha happy in the castle because he did not want him to see all of the suffering in the world. He did not want him to become a religious leader
The Buddha's father eventually let him go out. When he went into the town he did not see all of the luxuries he had in the palace. He saw suffering, with humans as the cause
After seeing this suffering Buddha went and sat under a tree and meditated about all the problems of the world
So the Buddha set out on 4 journeys to reach enlightenment. On each of these journeys he found that there are 4 noble truths in the world. That suffering happens, humans are the cause of suffering, suffering will end, and there is always a path out of suffering.
After taking these four journeys Siddhartha Gautama became the Buddha or enlightened one, starting what is now Buddhism.