It seems that, in any of the studies that you see about this book of the Bible, the focus is always on the "big fish". In reality, the "big fish" is a small part of the account of the story and the fish was merely a tool that God used.
"The word of the Lord came to Jonah son of Amittai: 'Go to the great city of Nineveh and preach against it, because its wickedness has come up before me.'" NIV translation
Many have tried to write this book of the Bible off as fiction but we are given a very real name in this first verse.
He is also mentioned in 2 Kings 14:25 where we are told that he was not just an ordinary man but a prophet. Nineveh was the capitol city of the Assyrian Empire at this time and the Ninevites were known to be brutal.
Their reputation for brutality even caused entire towns to commit suicide rather than to fall into their hands. The site of Nineveh is Northern Iraq near the modern city of Mosul. We see in this passage that they were in fact so wicked that God took special notice of them and told Jonah to go and preach against them.
"But Jonah ran away from the Lord and headed for Tarshish. He went down to Joppa, where he found a ship bound for that port. After paying the fare, he went aboard and sailed for Tarshish to flee from the Lord." NIV translation
We may think that he was scared to go to Nineveh and to speak against them. They were known for their cruelty and it would have been natural to be scared but that is not the reason that he chose to disobey God. The real reason is that he did not think they deserved salvation and was afraid that they would repent and God would forgive them. He hated them for their cruelty but another reason is that God had never sent a prophet to the nations that surrounded Israel. This was not something that was normal for a prophet to do because, until this point, the prophets were for the nation of Israel. The Ninevites were the first Gentiles (non-Jews) that God sent the message of repentance and forgiveness to and, even in the days that Jesus walked the earth, that was a hard thing for the Jews to accept. Instead of obeying the Lord, Jonah was fleeing to Tarshish. That was just about as far away from Nineveh as you could go as it was clear on the other side of the sea. We can see a very valuable lesson in that Jonah was called to go to his enemy and tell them how they could be saved from God's wrath. Jesus was sent here for us (His enemy) to tell us how we could be saved. Who do we consider to be our enemy and is God calling us to go to them and tell them of salvation? We also see a very good example of God allowing us to use our "free will" which was a gift from God. He could have stopped Jonah from paying the fare, getting the ticket, and boarding the ship but God let him go and learn the hard way.