Reference resources are a great place to begin your research. They can help you define terms as well as to find basic information about a topic, a chronicle of its history, theories, key people and sometimes a bibliography of additional sources.
There are many different types of reference sources, which include dictionaries, encyclopedias, thesauri, bibliographies, handbooks, maps, style manuals to name a few.
The OED is the most comprehensive and authoritative dictionary of the English language.
Even though most Wikipedia articles themselves aren't yet considered reliable enough as sources for a research paper, the References and External Links at the END of the articles may still help you.
Check the University of Pittsburgh Library's Wikipedia guide on the pros and cons of using Wikipedia.