When researching the law, it is important to remember the four main types of law: constitutional, statutory, administrative and case (common) law.
The Constitution of the United States is available several places, completely online: National Archives, FindLaw. You can also find the Constitution of the United States: Analysis & Interpretations, which contains references to US Supreme Court cases, via GPO Access.
Both state and federal statutes are available in print and online from Andersen Library. The federal statutes can be found in the Statutes at Large (in chronological order of the date of passage) or in the United States Code (codified by subject matter). The latest edition of United States Code (2006) is available in the Government Reference Collection. As for Wisconsin, you can find the latest edition of the Wisconsin Statutes both online from the Wisconsin State Legislature or via Westlaw.
The regulations passed by both the federal and state agencies can be found in publications online and in print at the library. For the federal agencies, check the Code of Federal Regulations, updated yearly. If you're looking for proposals from federal agencies, you'll find them in the Federal Register, published daily. Wisconsin has similar publications, the Wisconsin Administrative Code and the Administrative Register.
To find case law, you need to look in reporters. The place in which the trial was heard will determine the reporter series that contains the case. The US Reports or the Supreme Court Reporter will features cases from the US Supreme Court. If you're looking for a case that took place in Wisconsin, you have two options: the North Western Reporter, which has cases from Wisconsin and six other states, and the Wisconsin Reporter, which only has Wisconsin cases. Andersen Library has these titles in the Law Reference Collection on the second floor.
Fortunately for you, these reporters are also available online through Westlaw.