how did Manasseh began his reign.

The way of how King Manasseh started his reign remains me to the reign of King Solomon, but exactly the opposite way.
Solomon started as a righteous king, later on he forgot about the Lord his God, and became an evil King by disobeying God’s orders. After King Hezekiah died, his son Manasseh took over as a king, and had everybody worshiping pagan gods and idols, forgetting about the true God, the God of his fathers. As a punishment, God let the Assyrians attack Jerusalem and King Manasseh was carried out off Jerusalem chained, but after this something amazing happened, according second Chronicles, chapter 33, verse 12 to 13:

The LORD spoke to Manasseh and his people, but they paid no attention. So the LORD brought against them the army commanders of the king of Assyria, who took Manasseh prisoner, put a hook in his nose, bound him with bronze shackles and took him to Babylon. In his distress he sought the favor of the LORD his God and humbled himself greatly before the God of his fathers. And when he prayed to him, the LORD was moved by his entreaty and listened to his plea; so he brought him back to Jerusalem and to his kingdom. Then Manasseh knew that the LORD is God.

Now the evil king had discovered who is the true God, the God of his fathers, so he repented and became a righteous king and lived for many years. It is really interesting the similarities that Manasseh has with King David, because like David, Manasseh after committing huge sins against God, he repented hard core and became, as said before, a righteous king.
Like Manasseh, let us convert to God, let us stop worshiping the ways of the world, and become true sons and daughters of God.