The Greek epic poem the Odyssey, attributed to Homer, is, along with The Illiad, the oldest extant piece of Western literature. As such, Homer laid the groundwork for all future epic poems, including the Aeneid, Paradise Lost, and The Faerie Queen.
Every epic needs an epic hero; in the Odyssey, it is Odysseus, the King of Ithaca. He is not an ordinary man but a king who has performed heroic deeds (fighting in the Trojan War) and is favored by the gods. The gods are another important aspect of the epic or, at the very least, some supernatural element. In the poem, the gods and goddesses are active participants and make periodic appearances. The epic should, in some way, express the values, beliefs, and culture of its society, which the poem does. It is a repository of the Greek culture of the period, which believed in gods, in fate, and in heroism and bravery.
Another convention of the epic is that it needs to be on a large canvas. Though in our own time, "epic" is often synonymous with "long," an epic needs to cover both space and time. In the Odyssey, the hero and his crew range all over the Mediterranean, following the Trojan War. The action also covers many years. Two technical conventions that an epic has are the invocation of the muses, which starts the poem, and that it begins the action in medias res, or in the middle of the story.