These online dictionaries and encyclopedias are not specifically art-focused but may lead you to useful information or other resources.
Sometimes you need to learn about the life of a particular artist, designer, or photographer. Here are some resources that provide that kind of information.
When you start writing your paper, there are resources that can help, such as this book: Short Guide to Writing About Art (N7467 .B37). The Online Writing Lab at Purdue (OWL) gives guidelines and tips on formatting your papers and citations in APA and MLA styles.