The United States Code is the official printed code of the United States. A "code" is a topical arrangement of all the laws currently in force. While these are the current, official statutes of the United States and are the preferred set to which you should cite, this set is frequently several years out of date. You'll probably find the set behind you, "United States Code Annotated" easier to use. (ALWD rule 14.1(b) and Bluebook rule 12.3)
The U.S. Code is arranged by topic in "Titles", and within each title they are further subdivided into Sections. Use the index at the end of the set to find relevant topics and sections.
See your ALWD rule starting at 14.0 or Bluebook rule 12.3.