Today, it seems everyone has an opinion about everything. When we begin a research assignment with a formed opinion or argument in mind, it can be difficult to avoid bias in our writing. The choices we make when we look for information can be biased too. Looking for sources that support your thesis is an example of confirmation bias, and confirmation bias is, "a tendency to look first for information that confirms a desired conclusion" (Cornell Law School).
How can you avoid bias? Try starting with a question. Or, if your professor asks you to start with a thesis, turn it into a research question for your own use. Begin with the question, and let the evidence guide you to a thesis. As you read and take notes, your thesis should become more detailed, and it may even change. Be sure to create it based on strong evidence from a variety of sources and voices, not just the evidence you agree with.
Examples of strong research questions:
Content borrowed from Behrens, Laurence and Leonard J. Rosen. "The Research Question." Writing and Reading Across the Curriculum, Pearson Longman, 2005, p. 185.
Try opening one of these links to "shop around" for ideas. What topic do you wish you knew more about? What are you curious about? Or, as Wayne C. Booth put it: "find a topic you care about, ask a question that you want to answer."