Your professors may have mentioned that you should not use an encyclopedia to write your paper. This doesn't mean you shouldn't look at them at all. Reading an encyclopedia entry can help you become familiar enough with a topic to narrow it down. The other really important thing to remember about encyclopedias is that there are many kinds. Encyclopedia Britannica or the World Book are great places to start looking for information but if your topic is rotary engines don't forget to look for for an Encyclopedia of Science and Technology.
Textbooks are a great source for gaining a general understanding of a topic.
Also look for Handbooks , Yearbooks , Manuals , Almanacs , and Atlases are other sources to gain a fundamental understanding of your topic. Many of these are available online as well as in the reference section of the library. Look for Resources Guides accessible from the library's Web site or in the library. These guides are prepared by subject specialists and can particularly valuable aids for getting the information you need. I know how to use the Library catalog to find books and other materials on my chosen topic.