If the law you are investigating was a catalyst for new agencies or new regulations within existing agencies under the Executive branch, include also:
If at any time after the bill has become a law the constitutionality of the law may be called into question, you may find reference to a Supreme Court ruling. This is uncommon for the majority of laws. Lower court cases which help refine and define the law may be more common. None the less, the third branch of the federal government -- the Judicial branch -- is also responsible for creating law in the form of court decisions. Any major court case where a ruling affects a law should be included within a complete Legislative History.
In the case of a public law that creates programs which must in turn be carried out by the individual states, be sure to check state law (legislative branch) and state regulations (executive branch) for a complete legislative history.
When selecting a law for purposes of learning how to do legislative history research, try to avoid some of the larger and more controversial laws. They have been amended and changed so many times over the years that the amount of documentation is overwhelming, especially so for a learning example.
An example of a law to avoid is Social Security. The original law was the The Social Security Act, Pub.L. 74–271, 49 Stat. 620, enacted August 14, 1935 under Roosevelt. It was amended under Johnson as he Social Security Amendments of 1965, Pub.L. 89–97, 79 Stat. 286, enacted July 30, 1965. A complete legislative history of social security in the United States would include all pertinent documents from all laws and amendments since 1935.
Most legislative documents created prior to 1975 are not currently available in digital form (though this is changing constantly as more and more of the older documents are digitalized and made available online), and must be obtained by driving to a federal depository library (such as the Government Documents Library of the University of Michigan in Ann Arbor) to research and photocopy the print version.
Choosing to research such a complicated and controversial law will double (or quadruple) the amount of work necessary to complete an assignment that is designed to simply teach you HOW to do a satisfactory legislative history.
Also be aware that any law which generates a federal program that establishes departments be created in each state to oversee the running or enforcement of said federally mandated program will require you to track down many state government documents, a process not without its own problems.
Choose your program -- and public law -- carefully and remember the prime purpose of this exercise is to learn what steps must be taken and how to navigate through the documentation necessary to for a complete legislative history. Should the time come in your career when you find yourself in the position of needing to do this work on a specific issue (law), you will be prepared. But at this point, please try to avoid getting bogged down in endless reams of documentation generated in any complicated and/or controversial law.
Instead, search for something that you find interesting. It is often useful to design your research around something about which you have no prior knowledge, further enhancing the learning experience.
If, after selecting the law you wish to research, you suddenly realize the scope is larger than you originally assumed, consider changing to a different law. Wait too long, and the amount of work done but wasted will equal the amount of work yet to do on a new law.
When in doubt about any particular law or program you wish to research, please check with your instructor -- or with a librarian.
Good luck, and good hunting!