Siddhartha lives in a great palace in India and lives with everything given to him. He invites his friend to live in the palace but he has to leave and Siddhartha goes with him.
On the outside of the palace he finds a dead man, a sick man, and an old man. He asks his friend can that happen to me. He responded yes that all that could happen to you. Siddhartha realizes that all life is suffering.
After that encounter, Siddhartha decides to strive for enlightenment. He tries to reach that by fasting and praying for days on end. At the end, he realized it wasn't working so when a traveler offers him rice he eats it.
After he eats the rice he sits under a tree to rest and meditate. At that point, he reached enlightenment and created the eightfold path as well as the middle way. The middle states to not go to any extremes to end suffering in life. The eightfold path is a set of rules that came with Buddhism.
After that, the Buddha teaches other people about Buddhism and it grows and spreads throughout Gupta India.
Buddhism spread through trade routes to China and all the other major civilizations. The reason that Buddhism is so popular is that unlike Hinduism Buddhism accepts anyone of any race, age, or social class.