The Census Bureau is the leading source of quality data about the nation's people and economy. You have two options to start your search on the landing page of data.census.gov: a free-form single search bar or an advanced search filtering experience.
American FactFinder (AFF) will remain as an "archive" system for accessing historical data until spring 2020.
Video tutorials for the US Census Bureau can be found here.
You can also view it, and other statistic and demographic-finding tools, at the Social Work Library Guide.