Here are a few ebooks that will help you find out more about the legal system of the United States before delving into research.
Canada and the United States: Differences that Count by 
American Law: An Introduction by Consult the US Library of Congress' Guide to U.S. Federal Law and its Guides to Law Online for all states, which feature guides for researching laws, regulations, court decisions and secondary sources for federal government branches and all U.S. states and territories.
The US Constitution is central to an understanding of the American legal system. The original 1789 Constitution consists of the Preamble and Articles I through VII. The original Declaration of Independence is not a constitutional document. In 1791, Amendments I through X were added to the Constitution. The first ten amendments are collectively known as the Bill of Rights. There are, at present, 27 amendments to the US Constitution.
Originally the Bill of Rights only applied to the federal government, but as a result of how the cases interpreted Amendment XIV, these rights also bind the state governments.
Freedom of speech is contained in the 1st Amendment; due process is mentioned in both the 5th and 14th Amendments; equal protection in the 14th Amendment is interpreted to be included in the 5th Amendment; search and seizure is listed in both the 4th and 5th Amendments, and has been incorporated as part of due process in the 14th Amendment.
The United States has two separate and distinct court systems that exist side by side. The US Constitution, Article III, section 1 states that the judicial power of the United States shall be vested in one Supreme Court, and in such inferior courts as Congress may authorize. Thus Congress can legislate the existence and jurisdiction of federal inferior courts below the US Supreme Court level. Under the general residual power left to the states by the Tenth Amendment, all other cases are tried in the state courts.
In order to research US law, one must have an appreciation of both the federal and state court systems. Many legal materials are organized around this distinction. The federal and state court systems are described in greater detail in the following two sections of this chapter.