He was originally a servant of king Solomon, ruler over the house of Joseph (1 Kings 11:28). The Lord used him to rise up against the king because of his wickedness (1 Kings 11:26).
The prophet Ahijah met with him and prophesied to him of how he would draw the ten tribes away and become the king of Israel (see Two Kingdoms for notes on Ahijah's prophecy). He also said that if he followed the Lord, that he would be blessed. However, he turned to idol worship, and Ahijah cursed him instead (see 1 Kings 14:7-16 for the entire prophecy).
As part of his wickedness, he put down the actual priests of God, and assigned people himself (who were not of the tribe of Aaron). Read 1 Kings 12:31 and 2 Chronicles 13:9 closely to see this happen – remember that Moses and Aaron were both of the tribe of Levi, so removing those meant having people who were not descendants of Aaron.
He also set up idol worship through the land (1 Kings 14:15).