Any information that doesn’t fit within the framework of your outline, and doesn’t directly support your thesis statement, no matter how interesting, doesn’t belong in your research paper. Keep your focus narrow and avoid the kitchen sink approach. (You know, the one where you throw in every bit of interesting research you uncovered, including the fungal growth in the U-joint of your kitchen sink?) Everything you learn may be fascinating, but not all of it is going to be relevant to your paper.
If you choose a topic, begin researching, and realize that it isn’t the right decision for you for some reason, don’t fret! Although it requires a bit more time, you have the ability to change your topic even after you begin researching others.
Who would be reading this paper, should it be published? Although you want to write for your professor or other superior, it is important that the tone and focus of your paper reflect the audience who will be reading it. If you’re writing for academic peers, then the information you include should reflect the information you already know; you don’t need to explain basic ideas or theories. On the other hand, if you are writing for an audience who doesn’t know much about your subject, it will be important to include explanations and examples of more fundamental ideas and theories related to your research.
Whenever possible, look for peer-reviewed empirical research. These are articles or books written by experts in your field of interest, whose work has been read and vouched for by other experts in the same field. These can be found in scientific journals or via an online search.