Determine the dataset that holds the information for which you are searching. Then decide what information you want from this dataset. The Census Bureau has already made available many datasets and is adding more almost monthly for querying with the API tool. Currently, the most popular aggregate datasets to query for variables using the API are as follows:

• American Community Survey (ACS): ACS 1-Year 2011-2017; ACS 3-Year 2012, 2013; ACS 5-Year 2009-2017; ACS Supplemental 2014-2017; ACS Migration Flows 2009-2016
• Economic Indicators Time Series
• Decennial Census (SF1/SF3) 2010, 2000, 1990
• Economic Census 2012, 2007, 2002
• County Business Patterns (CBP) 1986-2016 and Nonemployer Statistics 1997-2017
• Population Estimates and Projections 2010-2018    
• International Trade (annual 2005-present, monthly 2013-present)

The complete list of all available datasets for the API is located here:

api.census.gov/data.html