There are 3 branches of government at the federal level: Executive, Legislative, and Judicial. This page provides shortcut links to pages in the US Government Manual for looking up URLs, office holders, and contact information.
Remember, however, that there are commissions, agencies and more to research in subordinate organization levels in the government.
Start with USA.gov and the US Government Manual, "...the official handbook of the Federal Government." Knowing your document's issuing agency (for example) greatly helps in being able to locate and access it!