Here are some links to handy, general Social Studies research sites:

PBS
Type your topic into the search box to access a wealth of information about tons of topics

The BBC
Type your topic into the search box on the upper right--like PBS, you can trust this site

Mr. Donn's Social Studies site
A great starting point, especially for 5th and 6th grade students

Social Studies for Kids
This is a great place to start a project--but ignore the ads!

Ducksters History for Kids
Click on the icons or use the search bar on the upper right

Google
Endlessly useful, this is everybody's go-to search engine, but always evaluate your hits before accessing them. Aim for credible websites, and avoid commercial ones. Not sure? Ask your teacher or parent.