The Early Life of Zeus
Zeus was not always the ruler of the gods. Actually he's a fourth generation ruler preceded by Chaos, Uranus, and his father Cronus. The first, Chaos, was basically the universe. Not much is known about Chaos. My understanding is that Chaos is just simply the origin of all things. The second was Uranus (The Heavens) who was overthrown by his son, Cronus. Uranus was a hateful ruler. He hated all his children which greatly angered his wife Gaea (Mother Earth).
Cronus overthrew Uranus by cutting off Uranus' genitals and casting them into the sea. The way that story plays out is that Gaea enticed Uranus to bed with her and when he got close enough, Cronus came out of hiding and cut Uranus' genitals off with a razor sharp jagged sickle. It is from this incident that many other deities were born. From the blood that dripped off the genitals came the Erinyes. When the genitals fell into the ocean, Aphrodite was born from the foam bubbling of the ocean. Ok, brief lesson in the Theogony is over. Back to Zeus' history.
Cronus had 6 children of his own with his sister/wife Rhea. Cronus was warned that one of his children would someday overthrow him, so he swallowed each of the first five children right after birth. Upon the birth of the sixth, Rhea substituted the child for a stone wrapped in clothing. This sixth child of course was Zeus. He was hidden in Crete until he was full grown, raised by the titan Gaea. As an infant, he was suckled by the goat, Amalthea. When he finally became an adult, he and his mother Rhea devised a plan to force Cronus to throw up the other five children. Zeus did this by dressing as a cupbearer and serving Cronus a special elixir created by Zeus' first wife, Metis. Zeus, with the help of his brothers, sisters, and the hundred arm giants, overthrew Cronus and the Titans after a 10 year war. The final victory was achieved when Zeus brought into play his trademark lightning bolt. He then banished Cronus and the rest of the Titans to Tartarus, the lowest region of the underworld.