In order to get a successful search going, you are going to want to make a list of related words after you narrow down your topic. So, for example, if you wanted to search about Celtic Mythology and Warfare you want to narrow down to a part of the Celtic culture (Irish, Scottish). The more specific you can get, generally, the better your results will be.
Wikipedia isn't a reliable source, but it is a good place to get hints on keywords to use. If your initial search isn't turning up, make sure to try synonyms before giving up.
Remember to use the tools that these sites both give you- save the sources so that the programs can format and alphabetize it for you. Make sure that you look over any autofilled-in information.