Start by using the Library's Online Catalog and enter your search terms, i.e. names, subjects, agency name, phrases, etc. into the search boxes to find items in the Gov Docs collection.
NOTE: These documents will have Superintendent of Documents, or (SuDoc) call numbers.
(ex.C 3.223/13-2:970/1-4)
TIP: SuDoc call numbers, like the example above, have slashes "/" and colons ":" as part of the call number. This is how they differ from a Library of Congress call number.
ALSO NOTE: Some Government Documents may be located in the general stacks and will have Library of Congress call numbers.
Here are various search engines for locating government information, both current and archived.