For U.S.-focused research, public, private and academic libraries and archives are a great source of reliable primary source documents. Many will have digitized archives accessible to the general public.
It is important to be aware that available materials and collections may not be representative of all voices in this country due to historically exclusionary practices in the collection, preservation and digitization process.
This is the hierarchy of government-funded libraries and archives in the U.S.:
FEDERAL > STATE > COUNTY > CITY/TOWN
I like to start with the most local resources and work my way up.
A repository of U.S. federal government documents. The topics these records cover can be relevant to states, cities and neighborhoods, but would have been issued by federal branches of the government.
The official national library of the United States, LOC has many digitized collections of archival materials, including images, photographs, maps, records, and much more.