To begin, you'll need to get an overview of your topic. A general or specialized reference source (an Encyclopedia, Almanac, or Atlas etc.) can be a great place to start. Consider your search terms carefully. Make a list of them. Make sure you have thought of broader and more narrow terms. Also remember to think about synonyms for your search words. Different search words will get different results! For example, try searching "deities" if "gods and goddesses" doesn't work. Or, try "religious ritual" in addition to "worship rituals." The more search word possibilities you have the better are your chances of success.
Once you have a general idea of what's at stake in your topic (context, major players, historical significance etc.), you'll be ready to look at it in a little more depth. Books (in print or digital) will give you depth and breadth. Begin with the Bentley School Library Catalog (Upper School)