Read and learn the U.S. Constitution to know exactly how our political system works. Learn the process by which Congress passes new laws. This is a brief but useful synopsis provided by C-Span. After that you should read How Congress Works for a more in-depth discussion of the topic. Refer to the Congressional Glossary periodically to learn any terms that you don't understand.
Agencies have many diverse powers for implementing national policy.
Congressional committees are vitally important in the legislative process, because they effectively decide whether or not a bill will go to the floor for debate. Executive Branch Websites has links to just about every executive branch department and agency that there is. House Committee Websites has links to websites for all the House Committees. This page also allows you to look up membership, subcommittees, schedules, hearings, juridsiction, and hot topics for the different committees, as well as a keyword search. Senate Committee Websites has links to all Senate Committee websites and gives upcoming meetings and hearings.