A legislative history is an examination of the documents created during the process by which a bill becomes law, and is sometimes used by courts to find legislative intent if a statute is vague or ambiguous.
The best way to begin is to see if someone has already compiled a legislative history by bringing together all of the documents or has at least listed and cited to the relevant documents. For more detail, see the Finding Compiled Histories section of this guide.
If there is no existing compiled history, you can start from scratch and find the documents on your own. For more detail, see the Finding Congressional Documents section of this guide.
These are among the best web sites for finding Congressional documents and related legislative history information.
(Note that for some, access is restricted to the USF community.)
These sites have lots of good tips and advice on doing federal legislative histories.