The British at the time controlled the Colonies of America. Britain had a Prime Minister and Representatives that represented towns and cities in the UK. This gives the British citizens a voice in their taxes, economy, government, ect. The colonists had no such representation, and they didn't have a voice in their government.
At the time, Britain just fought a war with France and some Redcoats had quartered in the colonists houses, for free. Then the British Parliament imposed a tax on the colonists. All of these angered the colonists. What fueled the war even more was the murder of 5 Bostonian which John Hancock labeled as the Boston Massacre. Then, Son of Liberties dressed up as Mohawks threw tea into the Boston Harbor because the UK forced the governor to pay for the tea with the colonist's money. This is known as the Boston Tea Party. Soon, King George III (who really was mad) sent troops to take the weaponry in Concord and Lexington which resulted in the Midnight Ride of Paul Revere (and William Dawes and Samuel Prescott) which resulted in the First shots at Concord or Lexington and the rest is history.