From their first appearance in historical literature around 100 BC, the Germans were polytheistic. They believed in many gods, just like the Greeks and the Romans and the Persians and the Indians and other Indo-European people. In fact, the most important German gods are closely related to the Greek and Roman gods. All three sets of gods come from an original Indo-European religion.
Like the Greeks, the Germans believed that the world started with nothingness. One god formed out of the nothingness and was the parent of the other gods. The Germans called this first god Twisto, or Ymir (depending on the region). Ymir was fed by a cow, and this cow made a man by licking a salty block of ice. From this man came the god Odin and his two brothers. As in the Greek story, Odin and his brothers killed Ymir. They created the world from his body. Ymir’s flesh became the earth, his bones became the mountains, and his blood became the oceans (the Greek Kronos also had his blood in the ocean). Odin and his brothers also made the first man and woman from two pieces of driftwood. They called the man Ash and the woman Vine (probably).
The Germans called the main part of the world, where people lived, Middle-earth. They believed there was a big ocean surrounding Middle-earth (as Germany indeed has the Atlantic Ocean to the west, the Baltic Sea to the North, and the Mediterranean to the south). Somewhere within Middle-earth, they said, was Asgard, where the gods lived. You got there by crossing the rainbow like a bridge. The world of the dead, Hel, was in the cold north somewhere. People sometimes associated it with a world of giants. These giants attacked fertility goddesses and carried them off just as Hades did to Persephone.
Learn by doing: Draw a map of Middle-Earth, Asgard, etc.
More about Odin
Bibliography and further reading about the Norse gods: