In the benging
So at first there were these two realms, Muspulheim, which was really hot, and Niflheim, which was really cold.
The lava and the ice-water stuff kinda just mixed together and made Ymir, this big giant dude. He spewed out all these dudes called the Jötnar.
Then a cow named Auðumbla came along and licked an ice-cube until it melted and it became Búri, who had Borr, who had three god-sons: Odin, Vé and Vili.
They were all like, "Dude. This universe is sooooooo boring!" So to get some action in they did something productive brutally murdered Ymir.
Then they built Midgard (the realm of the humans) out of his corpse: the seas were made out of Ymir's blood, the land was his flesh, the mountains were his bones, the heavens were his skull (isn't that a bone?) and the clouds were his brains (I will never look at a cloud the same way again).
So all the Jötnar were like, "Hey, can we join you?" And they were like, "No!"
So then Odin and Frigg (where did she come from?) had Thor, Vàli, Viðarr and Baldr. And everyone was happy! (except all the Jötnar)
Everyone spits in a bucket
Until they weren't. One day, the Aesir (a group of gods related to war, they lived in Asgard) and the Vanir (a group if gods related to nature, they lived in Vanaheim) got into a war.
Unfortunately, we don't know what caused it because the Vikings were bad at writing stuff down. The few remaining things they did write down were taken by Christians and BURNED (what the hell?!) and then re-written incorrectly.