Reference resources are usually encyclopedias, dictionaries, handbooks, manuals and other publications that you refer to when wanting a definition of a word or concept, a general overview of a topic, specific information about a certain period of time or event in history.
Selecting a topic is often one of the most difficult parts of doing research. If you have a choice of research topics, start by choosing a topic you like, or one you are curious about. Here are some more suggestions:
Is your topic too big? Is it too narrow? Will you be able to find enough on the topic? You won't know for sure until you get started, but developing a good keyword list, including some broader and narrower terms for your topic, will help and it's quick and easy to do. Start by doing some background reading.