God sent Jonah to Ninevah. Nineveh was an enemy of Jonah’s country, Israel. So, Jonah did not want to go to Nineveh.
Jonah tried to escape God. He tried to travel elsewhere, by ship. But there was a terrible storm. Jonah knew that God had caused the storm. So, he asked the sailors to throw him into the sea. But Jonah did not drown, because God sent a whale (large fish) to swallow Jonah. Then, Jonah prayed to God again. He thanked God, who had rescued him (Jonah 2). And God caused the fish to return Jonah to the dry land.
So, Jonah went to Nineveh. He warned the people that they must confess their evil deeds to God. Otherwise, God would destroy their nation. The people obeyed Jonah. So, God forgave the people in Nineveh. God did not destroy Nineveh at this time.
But Jonah was sad, because Nineveh was his enemy. Jonah wanted God to destroy Nineveh. But God told Jonah that God cares about the people from every nation.
The Book of Jonah is in the Bible (Old Testament).