Identify which king was ruler when the english civil war broke out

Charles I was the ruler in England when the English Civil War broke out.

The two sides in the English Civil War were the side of the king, Charles I, and the side of Parliament.   The king's side also leaned toward Catholicism or at least high-church Anglicanism, whereas the Parliamentary side leaned more in the direction of Puritanism.
The king was trying to exercise his authority over Parliament, especially in matters such as control of the army.  The king did have the support of higher ranking members of the House of Lords, who did not want to lose power to those in the House of Commons.  Catholics also supported the king, whose wife was Catholic and who had promoted greater rights in England for Catholics.
The Parliamentary side was led by Oliver Cromwell, a Puritan and staunch proponent of the powers of Parliament over against the king.  In the end, Cromwell's side won the war against the king.  The "Rump Parliament" (those members left who didn't walk out in protest) determined that Charles I had committed treason and they had him executed.