United States law can seem confusing to students used to studying the English legal system. Below I've listed some useful web sites which provide links to primary legal sources.
The Library of Congress provides legislative information via its Thomas service. This includes links to Bills, the Congressional Record, Presidential Nominations and Treaties.
The US Government Printing Office includes the online version of the US Code, and also links to the Supreme Court web site, which includes the opinions, orders, and other materials issued by the Court from 1991 onwards.
In addition, for state constitutions and case material the Findlaw service provides a useful list of links.