Image: Ganymede and the Eagle, by Bertel Thorvaldsen 1818-1829. Marble. Minneapolis Institute of Arts

Verily wise Zeus carried off golden-haired Ganymedes because of his beauty, to be amongst the Deathless Ones and pour drink for the gods in the house of Zeus -- a wonder to see -- honoured by all the immortals as he draws the red nectar from the golden bowl. But grief that could not be soothed filled the heart of Tros; for he knew not whither the heaven-sent whirlwind had caught up his dear son, so that he mourned him always, unceasingly, until Zeus pitied him and gave him high- stepping horses such as carry the immortals as recompense for his son. These he gave him as a gift. And at the command of Zeus, the Guide, the slayer of Argus, told him all, and how his son would be deathless and unageing, even as the gods. So when Tros heard these tidings from Zeus, he no longer kept mourning but rejoiced in his heart and rode joyfully with his storm-footed horses.

- Homeric Hymn V To Aphrodite 203

The most common version of the story of Ganymedes is that he was kidnapped while herding his flock on Mount Ida in Phrygia by an amorous Zeus, who came down in the form of an eagle or sent an eagle to carry him to Mount Olympus where he became cupbearer to the gods. An alternative version has Eos the dawn-goddess kidnapping Ganymedes as well as his brother Tithonus. Zeus then robbed Eos of Ganymedes, and in return granted her the wish that Tithonus be immortal (shame she forgot to ask that he also remain youthful...)

Some sources say that Zeus' affair with Ganymedes was a (religious) justification for homosexuality within the Greek culture, while others opine it's merely a reflection it. In his Laws Plato declared the Ganymedes myth had been invented by the Cretans who imported their tastes to Greece, and in Crete it was Minos rather than Zeus who was called the ravisher of Ganymedes. Maybe the connection with Crete arose because there's a Mount Ida there as well as in Phrygia? The Cretan/Minos association lingered however, and resurfaced in the late classical rationalisations of the myth in the Byzantine Suda, a massive 10th century CE Byzantine Greek historical encyclopedia of the ancient Mediterranean world:

"[Minos] hearing of the great fame, in Phrygia, of Tros the king of Troy and of his sons, he went to the city of Dardanos where Tros lived. Tros had three sons: Ilos, Assarakos, and Ganymede, [the last] of whom had a great name for beauty. So Minos stayed as a guest with Tros, both giving and receiving presents, and he ordered Tros to summon his sons, so that he might see them and give them presents too. But Tros said that they had gone on a hunt. [So] Minos too wanted to hunt with them. At first [Tros] sent one of his attendants into the place where the boys were hunting, around the Granikos river; but Minos, having sent out his ships a little beforehand to the river, came later to the sons and saw Ganymedes and fell in love with him. And having given out orders to the Kretans and snatched the boy, he him in the ship and sailed away. The place was called Harpagia. Minos took the boy and went to Crete. The boy to ease his pain killed himself with a sword, and Minos buried him in the temple. Hence, of course, it is said that Ganymede serves with Zeus." - Suidas s.v. Minos

Secondary source used: Theoi.com.