General Demographic Information:
An online data reference tool for the United States. It unlocks 220 years of U.S. Census data and makes them available in an easy to use fashion with data visualization, reporting and downloads.
See this page for tutorials: https://www.socialexplorer.com/teach-and-learn/training-modules
Don't forget about your local public library or historical society. They will have local history information and archives, often online. For example: Westfield (MA) Athenaeum has an easy to find section on their site strictly for local history resources. In addition to what's offered online, you can always call or visit the librarians, and they will be more than happy to help!