This problem is very common. Take a look at some background information on your topic (in a handbook or companion, encyclopedia, Wikipedia, etc.) and think about the different aspects of that topic--what's most interesting to you? The Topic Finder from Artemis Literary Resources is very helpful for this process.
Start searching your topic. This is the only way to get a feel for how narrow or broad it really is. If there are volumes and volumes written about your topic, you need to narrow it some more, but you need to find enough to inform and support your paper--it's a delicate balance. When in doubt, talk to your professor, but do so AFTER you have done some preliminary searching and background reading.
You usually reach this conclusion after 1) Your professor told you the topic is too narrow; OR 2) You can't find enough information on the topic. If you can't find enough information, go through the secondary sources guide to make sure you have covered your information options or contact your reference librarian. If your professor told you your topic is too narrow, try the following tip.
Search for your author, not just a specific work, then narrow from there. Or, pick a broader theme. Example, if you are interested in the color blue in a specific poem, search for books and articles about color and symbolism in literature, then try to narrow to your specific author. You may not find anything written on your EXACT topic. That's actually good news--you have original thoughts! Look at other scholarly articles to see how they pull together different pieces of scholarship--and parts of the literary text itself--to make their case.
Search for articles in a number of relevant databases in one easy search. Includes the MLA International Bibliography, Academic Search Complete, Communication & Mass Media Complete, and Education Source.