The World Book encyclopedias are a great starting place for research. You can also find primary source documents and even e-books here!
When you follow this link, you will be asked to choose a product. For most research, choose Advanced, which is designed for high school students, or Academic, which is designed for college students.
Specialized databases that you might find useful are Early Peoples (origins and rise of various civilizations and cultures), Inventions and Discoveries (objects and ideas that have shaped how we live today), and Living Green (impact of modern life on the environment).