Zeus was once married to Metis, a daughter of Ocean who was renowned for her wisdom. When Metis became pregnant, Zeus was warned by Earth that a son born to Metis would overthrow him, just as he had usurped his own father's throne.
So Zeus swallowed Metis. In time he was overcome with a splitting headache and summoned help from the craftsman god Hephaestus (or, some say, the Titan Prometheus). Hephaestus cleaved Zeus's forehead with an ax, and Athena sprang forth fully armed.
The poet Hesiod tells the story to account for Zeus's great wisdom, since he can be said to have literally incorporated Metis. One can also read into the myth wishful thinking on the part of the mythmakers who replaced the worship of the Great Goddess, mother of all growing things, for that of the male sky-god Zeus. Zeus gave birth to Athena himself, as if to say, Who needs a woman in order to bring forth new life?
Athena aided the heroes Perseus, Jason, Cadmus, Odysseus and Heracles in their quests.
Both Athena and Poseidon wanted to be patron deity of Athens. To prove her worthiness for the honor, Athena caused an olive tree to spring up on the citadel of Athens, the Acropolis. Poseidon sought to outdo her by striking the ground with his trident and causing a spring of water to gush forth. But as he was god of the sea, the water was salty. Athena's gift to the Athenians was considered to be more useful, so she became the city's patron deity.
Athena sponsored Perseus in his quest to slay Medusa because she wanted the Gorgon's head to decorate her shield.