The database Oxford Reference Online Premium includes several online encyclopedias that provide some great background information on your topic. If you begin by gaining some knowledge with these sources, you can get a better idea of the context and history of your topic, as well as some ideas for keywords to use in searching for more specific information later!
Especially useful is the The Oxford Companion to the Photograph.You can browse all the entries by first letter, but you probably want to use the "Search within work" option (look for it on the left) to find specific topics within photography that you are researching. Click on the image or the link above to try it out!
You can also click on "Art & Architecture" (after clicking "Subject" on the top left of the blue bar) to find many more reference books on art topics, with entries related to photography (look for the option to "Search within work" to find photo-related information in more general art encyclopedias).
After exploring options online, you may want to check out some background information in some encyclopedias and other books we have in PRINT in the Library. See below for books that cover general info, photography history, terminology, and biographical info on specific photographers.
Encyclopedia of Nineteenth-Century Photography by 
Encyclopedia of Twentieth-Century Photography by 
Photography Encyclopedia by 
A World History of Photography by 
Photographers: a sourcebook for historical research by 
The Glossary of Digital Photography by 
Looking at Photographs by 
A History of Women Photographers by 
Early Black Photographers, 1840-1940 by 
Macmillan biographical encyclopedia of photographic artists & innovators by 
The Photography Book by 