At the beginning of the research process, it's very useful to get an overview of your topic. Encyclopedias, dictionaries and other reference sources are a great way to get started and to find background information on your topic from various perspectives.
Gale Virtual Reference Library
A collection of online reference sources
Credo Reference
A collection of online reference sources
Colonialism: An International Social, Cultural, and Political Encyclopedia
An Encyclopedia of Western Colonialism since 1450
Europe 1789-1914: Encyclopedia of the Age of Industry and Empire
The British Empire: An Encyclopedia of the Crown's Holdings 1493 through 1995