If you have no idea:
Gail Levin Richmond, Federal Tax Research: Guide to Materials and Techniques (8th ed., 2010) KF 241 .T38 R5 2010 (Located at the Reference Desk)
Federal Tax Research explains where to find federal tax law. It also discusses publications which contain, interpret, and explain the law. It has a good index and a detailed table of contents.
The Internal Revenue Manual also has a section entitled “Researching Tax Law” 18.104.22.168.
The Library’s Major Treatises by Topic: Taxation