Theseus was the son of Aethra, daughter of the King of Troezen, and Aegeus, King of Athens and/or Poseidon. After the birth of Theseus, Aegeus returned to Athens, leaving his sandals and sword under a heavy rock, so that when Theseus was old and strong enough to move the rock, he would be able to take these emblems to Athens and claim his birthright.
At 16, Theseus moved the rock, took the sword and sandals, and travelled to Athens, killing six notorious bandits on the way. After several additional adventures, he saved Athens from having to send a tribute of youths and maidens to King Minos by killing the Minotaur.
Plutarch, cited below, is a major ancient source for the early life of Theseus.