The Constitution delegates the authority to pass legislation solely to Congress (the Legislative Branch); however, before a law can be enacted it must be signed by the president, and in order to remain it must stand up to judicial review.
A bill is a proposed law brought before the House and/or Senate for consideration. Members of the House and Senate may a propose bill to become law, although the majority of bills that come before congress have been drafted by interest groups and the executive branch. Bills dealing with money, taxes, and the budget must originate in the House, and only members may introduce bills in the House.
This page offers resources to help you find and track bills, as well as resources to help you to better understand the Legislative process.
These sources provide the full text of Public and Private laws as passed by congress. The United States Statutes at Large is the primary source for the laws and resolutions passed by Congress. "Slip Laws" are the official publication of individual laws, which are then compiled in the Statues at Large.
The United States Code is the arrangement of Public Laws into 54 subject areas, or "titles". The print version of the U.S. Code is published every six years, although online editions may be updated more frequently.
These sources either provide tools for identifying the law you are looking for or are guides to documents related to the Public and Private laws passed by congress.