The National Historical Geographic Information System (NHGIS), produced by the Minnesota Population Center at the University of Minnesota provides free access to historical Census information. Note: this resource requires user registration, but registration is free and immediate.
Census records can provide the building blocks of your research. The first Federal Population Census was taken in 1790, and has been taken every ten years since. Because of a 72-year restriction on access to the Census, the most recent year available is 1940. The 1950 Census will be released in 2022.
The National Archives has the census schedules on microfilm available from 1790 to 1940, and most have now been digitized by our digitization partners. Family researchers generally find it most helpful to begin with the most current census and work backwards as a strategy for locating people in earlier generations.