The US Constitution is the foundation of law in the United States of America. This is important because it gives something to fall back on to define how laws are interpreted. it is interpreted by the US Supreme Court. Article III of the Constitution lays out the organization and duties of the Supreme Court.
Article III Explained
U.S. Constitution | Constitution Annotated | Congress.gov | Library of Congress
First Amendment
First Amendment | U.S. Constitution | US Law | LII / Legal Information Institute
Second Amendment
Second Amendment | U.S. Constitution | US Law | LII / Legal Information Institute
14th Amendment
14th Amendment | U.S. Constitution | US Law | LII / Legal Information Institute