About Asgard: Asgard, even though inhabited, is physically different from Earth and other planets. Asgard is not a sphere-like shape, but a piece of land that floats in space. Asgard also doesn't rotate on an axis nor does it rotate around the sun, though there may be an object of some sort that gives it its sunlight. Gravity is similarly like Earth, although heavier. For example, a chair made of wood would be considerably heavier than one on Earth. What keeps the land together is unknown, but it keeps it together. Water doesn't pour from the land, as the gravity keeps it there as well. Asgard itself is connected to nine other dimensions which form the nine worlds of Asgardian cosmology through naturally occurring dimensional portals. Sometimes the nine different worlds have been treated as the same dimension with Midgard (Earth) being separate.
Creation of Asgard: Asgard's true origin is unknown due to the cycle of Ragnaroks. According to one story, originally there was a void, Ginnungagap, bounded on either side by fire Muspelheim, the land of fire and Niflheim, the land of ice. When fire and ice met, they combined to form a giant named Ymir and a cow named Audhumbla who nourished Ymir. She survived by licking the salty ice blocks. From her licking emerged Buri who fathered Bor and through him the Asgardian race. Bor took the frost giant Bestia as his wife and fathered three children: Odin, Vili, and Ve. Bor is attributed with founding the city of Asgard and being its first king. The three brothers eventually slew Ymir and from his body made the Earth and humanity. After Bor's disappearance and the deaths of his brothers in battle with Surtur Odin became the sole ruler of the gods.