The twelve olympians are the most important gods in Greece, they all live on Mount Olympus while ruling over the humans.  I will be explaining each of their origins and their subsequent rise to power. I will be starting with the big one of the big three Zues . 

Zeus 

Zeus was born from the two titans Rhea and Cronus. Cronus was told a prophecy that one of his children would usurp him so like all normal fathers he started to eat his children as soon as they were born. Very long story short Zeus grew up freed his siblings from Cronus’s stomach and they rebelled against him and won. The story of Hades, poseidon, Hestia, Hera, and Demeter is basically the same story as zeus. After zeus beat Cronus he took his place as the ruler of the cosmos.

The next people we are going to talk about are the six other olympians.

Dionysus+a little extra

Dionysus’s mother was a woman called Semele who was impregnated by Zeus (like always).  Hera, who was Zeus’s wife, was enraged by this so she shape shifted into an old lady and engracieted  herself to Semele.  Semele trusted the old lady immediately and told her that Zeus himself was her husband. Hera told Semele that all men use the i'm a god strategy to get in bed with a woman, so she should ask Zeus to show her his true form. Semele went to “Zeus” and asked him if he could show her his true form to her, so Zeus showed her and she was zapped into ashes. 

Even though she was ashes the baby she was carrying was somehow still alive so like any normal father he sewed the fetus into his calph so it could continue growing. After a very painful birth on Olympus the baby was probably going to be killed by Hera, so Zeus shipped him off to be raised in secret. Now Dionysus’s parents vary from the titan Rhea, Persephony, king Athamas  and queen Ino, and surprisingly Hermes. When he became an adult he traveled the world and learned how to make wine to also spread across the world. He came back to Olympus and took his place among the twelve great gods. 

extra bits:  one thing people did not agree on was that no one knew if Dionysus or Hestia was the twelfth olympian. depending on who you ask they will either say Dionysus is the twelfth or Hestia.   One other thing is that there are multiple gods that are different incarnations of Dionysus. Example Orphic Dionysus. He represented life, death and rebirth for the Greeks and cults would spring up to do sacrifices to the god. One of the other stories relating to orphic Dionysus  is one about orpheus.

Athena 

Before Zeus married Hera He had a wife before that and her name was metis she helped him during the Titanomachy by reuniting him with his siblings.  After the war Zeus received a prophecy by Gaea that Metis would bear children that would be more cunning than Zeus. So like any normal husband Zeus absorbed Metis and gained all her knowledge. Years later Zeus had a bad headache and slammed his head on a desk, so hard that his head split open  and Athena popped out fully grown.

Hermes+ a little extra

Hermes was born from Maia, one of the Pleiades and the god Zeus, in a cave on the island of Arcadia.  As soon as he was born he started to fool around. The first thing he did was kill a turtle and turn its corpse into a lyre.  After a while of playing with his corpse instrument he got hungry, so like any normal person he stole forty of Apollo’s bulls to eat, so he sacrificed some of the meat and kept the rest for himself.  After a while Apollo realized that his bulls were gone. Apollo then started to track down a baby Hermes and he did find him. 

Apollo then integrated the baby hermes and Hermes was just like i’m a baby what could I do with cows?  Apollo got so mad he took the baby to his dad, Zeus  and he found it really funny.  After a good laugh Zeus told Hermes to lead Apollo to the lost cows.  Along the way Hermes was playing his Lyre and he was playing it so well that Apollo was enamored with it, so much in fact that he promised Hermes that he would be the messenger of the gods. Apollo then gave Hermes his Caduceus.

Extra bits: 

There is another god that Hermes was an offshoot from.  The gods name was Pan his origins are mysterious because no one can agree on what his origins are.   Pan was the god of wilderness

fertility and a bunch of other stuff.    He was also the god of shepherds, flocks, things that Hermes was also the god of.  Pan was also an offshoot of another god called *péh2usōn who was pan but a little extra stacked on him like marriage, the sun and stuff.  

Other than Pan there was another offshoot of *péh2usōn and his name Pushan. After all this, people assume that Pan or something relating to him came before Mycenaean Greece and spread from Arcadia to places like India and some places in Europe.

Aphrodite + a little extra 

Aphrodite will be hard because she has a ton of epithets and many different forms of her and after the Theogony Hesiod does not mention how she met the other gods.  Aphrodite was born from the severed genitals of Uranus/Ouranos the god of the sky. After she was born she drifted onto the  shores of Kythera. That's all Hesiod wrote.

Extra bits 

Before Ancient Greece Aphrodite was not a thing but the closest thing to her was a god named Astarte and she was a Phoenician  goddess. She was brought over to Kythera by Phoenicians who built a settlement there.  She might of been one of the reasons that Aphrodite sprung up in the first place.

Artemis and Apollo

The story starts with their mother Leto, who was having an affair with Zeus and got pregnant. Hera knowing about this gave Leto a deal. She could not give birth on any island or piece of land that was touched by the sun. Leto somehow found this piece of land that was not

connected to the seafloor and was not technically touched by the sun since Helios never went there for some reason. On that little island Leto gave birth to Artemis and Apollo.

Ares

Sorry but there is nothing on this guy other than him getting kidnapped one time.

Hephaestus 

There is nothing much on this guy either but what I do have is his origins summarized. He was born to Hera and Zeus. Hera thought the baby looked so ugly that she threw the baby Hephaestus off Olympus. That's it.