Aigeas (king of Athens) went to Troezena (a city located SW of Athens) and met with (sorry, forgot the name), daughter of Troezena's king. When she got pregnant, Aegeas decided to go back to Athens. Before leaving, he covered his shield and sword under a huge rock and told her that when their son would grow up, he should move the rock and bring his weapons back.
Indeed. Theseus grew up and became a brave young man. He managed to move the rock and got his father's arms. His mother then told him the truth about who his father was and how he should take the weapons back to him. Theseus had to go to Athens and could chooes to go by sea, which was the safe way and by land, following a dangerous path with thieves and bandits all the way long. Young, brave and ambitious, Theseus decided to go to Athens by land.
Theseus faced every one bandit (...details needed...) and made it to Athens.
He did not reveal his true identity. He was welcomed by Aegeas who was suspicious about the stranger who came to Athens. But when he saw his own shield and sword he recognised his true son.
At the time, Athens had to pay a horrible tax to Crete. Six young men and young women had to be sent to Crete. They would put them inside the Labyrinth to be eaten by Minotaur. Theseus volunteered to be one of the six men. We the help of Ariadne he killed the Minotaur. (...death of Aegeas...)
Theseus was considered by Atheneans as the great reformer. His name comes from the same root with "θεσμoς - thesmos" greek for institution. As we can read in Aristophanes' "Frogs", they used to credit their everyday's habits to him.
(Needs re-writing. Will soon)