Decide on a research question for your paper. The research question is the unsolved problem at the heart of any research paper. Even if you do not intend to phrase your title as a question in the final version of the paper, write it in question form while planning out the paper.
Make a list of the information you will need to answer your research question. What information will you need to verify or disprove your hypothesis?
Assess each source of information as you read it, deciding whether or not it is relevant to the research question. Compare each source you find to your research question and your list of needed information. Don't be led astray by fascinating but irrelevant information. If you have interesting ideas to explore that don't fit into the structure of this paper, make them the basis of a future paper rather than forcing them into this one.
Reread your paper when it's finished. Did every source you cited address the research question or contribute to addressing it? Edit out sections of the paper which are not relevant.