The ancient Sumerians were the first people to develop an actual written language. We call that writing cuneiform.
The ancient Sumerians kept records of everything. Written records were very important to them. They needed young men to learn to read and write. Their language began as pictographs just like the Egyptians and Chinese. But pictographs are not easy to form into sentences, so they started to use triangle shapes to represent sounds, which they could then use to spell words.
They did not have paper or ink, so they used tools made of wood or stiff reeds to press the symbols into clay tablets. Many thousands of these have survived the ages and we can see them today. Some are worn, some are broken, but many thousands have been found complete. The largest collection of these was found at the Library of Nineveh. There we found complete stories like The Epic of Gilgamesh, which is the oldest written story in the world. The Sumerians invented Cuneiform, but the Babylonians and Assyrians continued to use it long after the Sumerians had been destroyed.
Cuneiform is not easy to read today since through time the meaning of some of the symbols changed as language changed. With patience and effort, it can be deciphered. There are many cuneiform tablets that are still waiting to be translated and ancient tablets are still being found today.