John was the youngest of both of his brothers, so he never really never thought that he would become king.
But when his brother died, John did become king.
Sadly John was a poor military strategist, he had to raise a lot of money and soldiers just to defend himself, he raised taxes to raise more money but he didn't ask permission of his Barons to raise taxes, which angered the Barons.
In 1251, the Barons rebelled, they marched into London, and took control. John then met the rebells at a famous field called Runnymede, and made a peace treaty called the Magma Carta.
The Magma Carta included:
even though they wrote a peace treaty, neither side took the peace treaty very seriously.