To begin, you need the following information:
1) the public law citation (ie. Pub. Law 101-336) or
2) the Statutes at Large citation (ie. 104 Stat. 327) and the bill number and Congress of the Act
These are the key access points as most sources are organized by one or the other, though some online sources will allow you to perform keyword searches.
Public law numbers have been used to identify enacted legislation since 1957. The first number indicates the session of Congress in which the law was passed and the second number represents the order in which the law was passed during that session.
Bill numbers can be found on the first page of each statute as it appears in the United States Statutes at Large since 1904; to locate bill numbers for laws passed before 1904, consult Eugene Nabors, Legislative Reference Checklist: The Key to Legislative Histories from 1789 to 1903
If you are starting with the popular name of the statute, consult a Popular Names table such as the one found in the United States Code, United States Code Annotated or the United States Code Service to find the public law and Statutes at Large citation.
If you are starting with a U.S. Code citation, consult the historical notes to find the public law public law and Statutes at Large citation. There may be several listed, in which case, you should record them all.