After the Revolutionary War, the new country had to decide how to govern itself. The Continental Congress wrote the Articles of Confederation in 1777. This document gave the new individual states power and put in place a weak central government. The Library of Congress has an easy to read timeline for the Articles of Confederation. This new system created lots of problems, and in 1787 all the states except Rhode Island sent delegates to Philadelphia to fix the Articles. Instead they wrote an entirely new document called the Constitution of the United States.
The Constitutional Convention changed the future of the United States. The delegates decided that their work must remain a secret. They argued and they compromised and they created the three branches of government still in use today.
If you want to know more contact a librarian.