At the start of the Revolution the people decided that they needed a government strong enough to rebel against England, and win.
Now that we are far away from England we need rules to follow.
In order to do this they decided to ratify the Articles of Confederation. The Articles of Confederation were designed to make a powerful central government, but essentially to have the states run by their own governments. This was bad for the central government because it doesn't allow the central government to collect money from the states. This left the central government weak, even though they needed a strong central government.
The Declaration of Independence was created in the middle of the turmoil between England and the colonies. The colonies wanted freedom so they created the Declaration of Independence which claimed their freedom from England.
We have finally won our independence from England!
We now need a stronger central government!!
The people realized that a stronger central government was needed if the U.S. were to stay on its feet. So Alexander Hamilton decided to call a Constitutional Convention. The Constitutional Convention was a meeting of high officials from each state to amend the weak central government. They decided to separate the government into three parts- The Executive Branch, the Legislative Branch, and the Judicial Branch.
The Executive Branch is the president, who has the power to enforce the laws. The Legislative Branch is also known as Congress and consists of the House of Representatives and the Senate and they have the power to make the laws. The Judicial Branch is the Supreme Court (and other courts) and it has the power to interpret the laws and to decide if they have been broken or not.