A prophet’s job is to tell people messages from God.
God told Jonah to go and tell the people of Nineveh that in 40 days’ time the city would be destroyed because of their wickedness. Jonah didn’t want to do it. 

The people of Nineveh were enemies of Jonah’s people, the Israelites.

So Jonah got on a ship heading for Spain. He thought he could get away from God!
While Jonah was asleep on the boat, a HUGE storm came up. The sailors were terrified! They threw cargo overboard and prayed to their gods. But the storm raged on. 
Finally they woke Jonah up and told him to start praying to his God. 
Jonah confessed that he was running away from God so that would be a bit awkward!
The storm was getting worse. Jonah told them to throw him overboard. 
He said to them, “I know this storm is my fault.’
They didn’t want to do it but in the end they ran out of options. So the sailors picked Jonah up and threw him into the sea. Straight away it was calm!
They were gobsmacked! They realised that Jonah’s God was IN CHARGE!!

What about Jonah? 
He nearly drowned. (read his prayer in Jonah ch 2, especially verses 5 & 7) But God sent a large fish and it swallowed him ……. without chewing! 
Jonah was inside the fish for 3 days and nights. WHEW!
Jonah prayed and prayed and prayed. Eventually he told God he would do what God wanted him to do.
So God told the fish to vomit Jonah up onto the beach ……. and it did!

Then Jonah went to Nineveh and spent 3 days walking around telling the people God’s message, and the people, even the king, believed it. They didn’t just say they believed it; their actions showed it. They did the things that God’s people, the Israelites, would do to show God they were sorry. (see Jonah ch 3 verses 5 - 9) 

You would think that Jonah would be happy about that, but he wasn’t. 
He said to God, “Didn’t I say that this is just what you would do? That’s why I did my best to run away to Spain! I knew that you are a loving and merciful God, always patient, always kind, and always ready to change your mind and not punish.”

God had to remind Jonah that he should have had compassion on Nineveh, especially the children and animals.