The Constitution of the United States of America
http://www.usconstitution.net/const.html
This is the supreme law of the land. Any laws contradicting it--at any level (federal, state, county, city, etc.) should be deemed invalid. This is not to say that the law cannot be changed if the people believe it was not written as it should be at the first; the document itself provides for this. Just as any laws violating the rights stated in the Constitution are invalid, so are any laws that are made at the federal level that pertain to issues not laid out in the document. (See the 10th amendment.) This was to limit the federal government from taking too much power. The federal government is there to protect us, nothing more. It is not to violate the sovereignty of the states respectively.
Read the Constitution and understand what it means. It's not hard to understand. However, if you do have trouble with legalese or with the language of the day, I recommend you read the Federalist Papers. Happy reading!
http://thomas.loc.gov/home/histdox/fedpapers.html