Even though in Greek mythology Zeus is the father of the gods, he is the sun of Cronos. Cronos is an offspring of Gala ( the earth ) and Uranus ( the sky). So this is how originally Zeus became father of the Gods.
Cronus is known for marrying his sister Rhea and together they produced the gods and goddesses of Mount Olympus including in addition to Zeus, Hera, Hades and Poseidon. These were the original Gods and Goddess of Mount Olympus.
Cronus took power over the heavens from his father Uranus by castrating him with a jagged-toothed sickle. Once he had killed his father he became aware of the prophecy that he would also face the same fate and die to the hands of his own sons. Cronos then took the matter into his own hands by swallowing each child that his wife produced.
Only the youngest God, Zeus was spared this as Rhea gave birth to him in a secret mountain, then presented her husband with a rock in a blanket to trick him as he swallowed him in one gulp.
Then when Zeus became of age the prophecy became true as Zeus killed Cronos with a jagged- sickle and released his siblings from his father stomach. This then paved the way for the next generation of god of Olympus. Zeus then later on Married his sister Hera.