I wonder...

How did Theseus' story begin?

Theseus' story begins with King Aegeus and his wife, Aethra, the parents of Theseus. Aethra became his wife by the means of her father, Pittheus, giving her over to his good friend Aegeus, because him, even having been married twice, had no heirs to the throne. After their secret wedding and having lain with the King, Aethra took a walk in the moonlight, which took her through the shallow waters of the sea to the Sferia island. There she met with the god, Poseidon, where he seduced her and took her on the exact same night that the king had. This was the result of Theseus' divine qualities. Soon after his son's birth, the king left to return to Athens, but not before giving Aethra instructions about 'his' son, the instructions he told her were to take Theseus, once he had reached manhood, and move the boulder the king had put his sandals and sword underneath. These items would prove useful later on to prove the boy's heritage. 
Once Theseus had reached the appropriate age his mother took him to the boulder and asked him to move it. Theseus completed this task with ease, lifting the boulder like a pebble, then fetching the sandals and sword of his fathers at his mother's request. His mother then told him that he needed to journey to Athens and meet his father. Aethra and her own father, who had helped raise Theseus, begged for him to travel to Athens through the sea, but Theseus denied them, taking the robber-and-thief infested roads instead, feeling no fear of the thieves and robbers that littered the road like trash.
Soon enough, Theseus encountered the first of his difficulties along his path, he had reached Epidaurus, a place sacred to the god Apollo and his son Asclepius. There he met the son of Hephaestus, Periphetes, known to bash the brains out of travellers with a club. Theseus was able to recognize Periphetes on site, as his grandfather, Pittheus, had described him to Theseus in detail. Periphetes, also known as The Cudgel Man, brandished a thick club, and upon seeing Theseus he told him of how he was going to kill him with the weapon. But, before he could strike Theseus began to complement the club. Periphetes, who often bragged, told the youth of how it was made entirely of brass. Though Theseus was not as dumb as he may have seemed, he countered how it couldn't have been made of entirely brass, proposing that it was actually made out wood, and covered in a sheet of brass instead. To prove him wrong, Periphetes handed the club to Theseus so he could inspect the club himself. As soon as he had the club though, Theseus delivered a strike to Periphetes' head, knocking him onto the ground straight away. Theseus decided to keep the club to use later.
Theseus soon encountered his next challenge, a man named Sciron blocked his path, demanding that to be let through, Theseus must wash his feet, claiming that the domain near the cliff's belonged to him. Curious, Theseus questioned the consequences of not doing as Sciron asked. Sciron soon replied that the consequence would be that he would cut Theseus' head off. Even so, Theseus could sense the man's weakness, and, smartly, agreed to wash the man's feet. As began to wash the man's feet, with the cliff face behind him, he looked into the water below him, seeing a gigantic turtle waiting at the bottom of the cliff face. As soon as he made the conclusion that this was the man who would most likely attempt to kick him down straight into the turtle's mouth, he grabbed onto Sciron and threw him down to the turtle instead.
The next challenge Theseus encountered included pine trees, even more attempted murder and a real murder. The man who caused such a challenge was none other than Siris, the Pine-bender (don't worry I've never heard of him either). As soon as the strange man looked upon Theseus he asked for help with an even stranger task, he asked Theseus to help him hold down a bent pine tree, but as soon as Theseus had a good grip on the pine Siris leapt back. The man expected the pine to catapult Theseus into the air, but he did not expect him to be able to hold such a tree down. Believing the pine must have snapped he walked behind the tree, but then Theseus let go, let the pine hit the man, knocking him out. Theseus then took two more pine tree's tying Siris' arms to one and his legs to the other, just as Siris had done to his own victims, letting him go to finish him off, his upper body being torn from his lower.
At this point, the sun had started to set, and it would soon be nightfall for Theseus. Thankfully, Theseus soon came across a house. Hoping the people inside would be kind enough to let him stay, he approached the home. After knocking on the door the man who answered introduced himself as Procrustes, and told Theseus that he had a magic bed that he could stay in for the night. But Theseus had heard of Procrustes and his 'magic' bed, the bed would fit anyone six feet perfectly, but if you were too short you would be bound to the posts and stretched to size, and if you were too tall you would be cut to size. But Theseus followed Procrustes to the room that held the bed. But as soon as they reached the room Theseus forced his host down onto the bed, cutting off his legs, and, to ease his pain, his head.
Travelling through the night Theseus soon reached the beautiful city of Athens. But, his father had married a sorceress named Medea, who predicted that Theseus would cause a problem between the king's relationship with her and how she wanted her own son, Medus to ascend to the throne. Because of her jealousy, she convinced Aegeus that Theseus had come to kill him, and suggested to invite him to a banquet and poison his wine, Aegus quickly agreed as he didn't recognize Theseus as his own son. Theseus eagerly attended the banquet, but as he went to drink the wine, Aegeus recognised the sword that hung off his belt, ordering for him to stop, finally recognising his own son. While Theseus and Aegeus celebrated at the discovery, Medea escaped, as to not be punished by the king.

https://greekgodsandgoddesses.net/myths/adventures-of-theseus/
https://www.greeka.com/attica/athens/athens-myths/theseus.htm
https://www.greekmythology.com/Myths/The_Myths/Theseus_Adventures/theseus_adventures.html