In the beginning, there was nothing. A lot of nothing. The first god, if you want to think of it that way, was Chaos - a soupy, gloomy mist with the matter of the cosmos just drifting around.
Eventually, Chaos got less chaotic (it probably got bored with being misty and gloomy). Some of the matter collected and solidified into earth, which developed a personality. She called herself Gaea. Gaea was the actual earth, the hills, rocks, and valleys, but she could also take on a human like form.
After a long time Gaea thought to herself about how nice it would be if there was someone else so she didn't have to be lonely. Either Gaea willed it to happen or Chaos cooperated with her but soon there was a big dome in the sky. It was blue during the day and black at night. He gave himself the name Ouranos and visited the earth often. Soon, they got married. Lived happily ever after? Not exactly...
The first set of kids they had together were called the Titans. There were 12 of them in total. 6 girls and 6 boys. Then they had 2 sets of triplets. Gaea named one set the Elder Cyclops and the others The Hundred Handed Ones. One look at them and Ouranos threw them down Tartarus and chained them there. Tartarus was a deep pit below the earth which Chaos had made. This made Gaea angry and she started to hatch a plan.
Gaea asked the Titans to kill their father. While everyone was debating if this is a good idea, Kronos, the youngest Titan, decided to kill his father with the scythe that his mother had made while his four brothers held him down. Each of the four brothers who held him down got the four corners of the Earth to rule.
Kronos then became the lord of the universe. Iapetus became the Titan of the west. Hyperion got the east. Koios took the north and Krios got the south. Since Oceanus didn't help Kronos took Ouranos's pieces and threw them into the sea. Kronos them released the Cyclops and Hundred handed one from Tartarus who then made Kronos a massive palace on top of Mount Othrys, the tallest mountain in Greece at the time.