If there is one place where the goals and rules for American democracy are written down, it is in the Constitution of the United States.
The best place to find an actual formal statement of the goals of American democracy is in the Preamble to the Constitution of the United States. The Preamble tells us why the Framers decided to create a new constitution. It says that they did so
in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity…
We can say, then, that these are the goals of American democracy. Since these goals are so broadly stated, they pretty much encompass everything that we want from our democracy.
The rules for our democracy are spread throughout the rest of the Constitution. There, we find rules telling us that we will have three branches of government. We find rules telling us much of what those branches are allowed and not allowed to do. We find rules saying how the members of these branches will be selected. These are all of the fundamental rules of our democracy.
There are other places where rules and goals are written. For example, many of the rules of our democracy have been set out in opinions of the Supreme Court. However, if there is one central source where we can find the rules and goals of our democracy, it is the Constitution.