There are a lot of datasets out there; your professor might prefer that you use one particular resource so be sure to talk to your professor before you begin.
Here are a few options:
What it is: Headquartered at the University of Michigan, this organization provides data on a wide variety of social science topics and allows you to contextualize the data with related publications.
Topics: Demographics, politics, education, consumer behavior
What it is: Datasets and visualization from government-funded research through the 2013 Open Data Policy.
Topics: Local government, climate, agriculture, education, energy, economic
What it is: Datasets, dashboards, and visualization from the US Census Bureau
Topics: Population size, demographics (age, sex, race), income, education level, and more. Information is available at the national, state, and county levels (depending on data type).
What it is: This resource, provided through the United Nations compiles data from international organizations on a wide variety of topics.
Topics: International Development, Trade, Education, Technology, Health
What it is: Community based repository for a wide variety of datasets (for analysis and machine learning)
Topics: Economic, Health, Education, Climate Change, Football
What it is: Data collected by NASA on a variety of scientific topics, primarily about environmental sciences. (Must register for a free account to Download)
Topics: Environmental Science, Chemistry, Biology
What it is: Provides overview articles that visualizes international trends. At the end of each article (before the footnotes), they provide links to data sets used in compiling the information.
Topics: Health, Politics, International Relations, Technology, Education, Energy, Environment