Go to your local library. Ask a librarian to assist you in finding age appropriate materials. For example, National Geographic has a series for children titled "National Geographic Countries of the World" that provides information and photos for many countries, including Greece. These books will be located in the Children's Nonfiction or Reference section of the library.
Search the CIA's online Fact Sheet on Greece. This is an invaluable resource when researching any country. The Fact Sheet provides details on Greece's history, culture, and politics, as well as provides maps and pictures, which is useful as visual materials in a book report.
Find information about Greece in an encyclopedia, Encyclopedia Britannica for example. The encyclopedia is a good place to start researching, because the information will be basic and not as detailed, and easy for a sixth grader to interpret. Look in the index of the "G" volume of the encyclopedia to find Greece.