Understanding Norse mythology is sometimes difficult, as there are many stories about Odin and all the mighty gods. But one Norse story that people rarely discuss is the creation story.
According to Norse mythology, the world began with Niflheim, the land of ice, and Muselheim, the land of fire.
The fire and ice met, creating Ymir, the first-ever giant. When Odin and his brothers killed Ymir, they constructed the world with his remains.
This article will go into more detail about this story and how it all began.
Also, see How Do You Learn Old Norse? to learn more.
In the Beginning…
Before man and gods, there was only Niflheim and Muspelheim.
These realms were the only things that existed. So, there was no grass or sky at this time. Niflheim was the land of frost.
So, it only held ice. Meanwhile, Muspelheim was the land of solely fire. These two realms existed with only a gap between them for a long time.
Eventually, the fire and ice began spreading further. As the fire from Muspelheim came closer to the ice of Niflheim, the gap between them began to close.
When the ice and fire finally met, the melted ice dripped onto the ground and formed Ymir. Ymir was the first being to exist in Norse mythology.
Ymir was a powerful giant who could reproduce asexually or by himself. So, after the fire and ice created Ymir, he began to create other giants, as he did not need a mate to procreate.
As the frost from Niflheim continued to melt, other beings started emerging from the ice.
First, there was a cow named Audhumla. She was the first to emerge from the melting ice of Niflheim.
She produced milk for Ymir and his offspring to drink. As the giants needed more milk from her, Audhumla began licking the remaining ice to keep herself nourished.
As Audhumla continued licking the ice for nourishment, she uncovered another being found buried in the ice.
This being was the very first of the Aesir gods called Buri. While Norse mythology does not provide a name for Buri’s wife, he was able to father a son who he named Bor. 
Buri’s son Bor married a giant named Bestla. Together they had three sons Vili, Ve, and Odin.
Unfortunately, but not surprisingly, not everyone got along.
In fact, it wasn’t long before Odin and his brothers decided to slay Ymir, the original giant.
Even after the creation of Ymir and the other gods, there was still only fire and ice that existed in the world.
Odin, Vili, and Ve set out to destroy Ymir and use his body to create the rest of the world. And so they did.
The three brothers hated the world as it was. There was no greenery or sky, just fire and ice.
So, they knew killing Ymir would allow them to use his body to form the world. After his death, Odin, Vili, and Ve created the rest of the world. 
The three brothers used Ymir’s blood to form bodies of water. They also used his skin to make the soil. Many parts of Ymir’s body made up the world as humans see it now.
Notably, Odin and his brothers used Ymir’s brain to make the clouds and his skull to form the sky.
After creating the earth, the gods went even further.
They made the first humans from tree trunks and placed them in Midgard to live.
Odin and the other gods knew that the humans needed protection from the giants that Ymir created.
So, they implemented safety procedures to keep humans and Midgard safe from the giants. 
Humans in Norse Mythology
According to Norse mythology, the world started with giants and gods. So, people may wonder where humans fit into all this.
In Norse poetry, there is a strong association between humans and trees. (This makes more sense with the understanding that the gods created humans from trees.)
Odin was with two companions when he created humans. Some records indicate he was with his brothers Vili and Ve, while others argue that he was with Hoenir and Lodurr.
Whoever he was with, Odin and the two other gods stumbled upon two trees.
The trees were lying on the ground, but their shape looked like human bodies.
Because of how they looked, the gods decided to give them life. So, Odin and his two companions gave life to the trees, creating a human man and a woman.
Odin breathed life into them while his companions gave them good health and sound minds.
Afterward, Odin dressed the humans and gave them Midgard as a new home. These two humans, Ask and Embla, ruled happily over Midgard and became the ancestors of the entire human race. 
The creation of humans in Norse mythology explains why many poems reference humans and trees almost interchangeably.
Since Odin and the gods created humans from trees, it makes sense that poems compare them so often.
The Nine Realms
After killing Ymir and using his body to fashion the world, Odin separated parts of the world into realms. The nine realms are the staple for understanding Norse mythology and how everyone coexisted.
When Odin came along, there were already two realms.
Muspelheim and Niflheim came about before Odin as the realms of fire and ice, respectively.
So, after the death of Ymir, Odin created seven more worlds for the people and gods to live in. Midgard was for the humans to reside safely away from giants who may harm them.
Odin gave his own people, the Aesir, their own home called Asgard.
The giants could reside in Jotunheim, the dwarves lived in Nidavellir, and the Vanir established roots in Vanaheim.
Odin also created a home for light and dark elves to live separately. He kept the light elves in Alfheim and the dark elves in Svartalvheim.
The creation of the nine realms leaves many people wondering why there were nine. Surely, Odin could have created more if he wished.
The number nine plays a significant role in Norse mythology, even outside the nine realms. So, this number was chosen purposely by Odin.
The article “Why Did Odin Stop at 9 Realms?” provides more information on why the number nine is so significant in Norse mythology.