Consider the following questions: How many sources do you need? What types of sources do they have to be? Can they be popular sources, or do they need to be scholarly? This can help you figure out what you need and where you can start searching. Google is great, but it's easier to find scholarly sources through the library databases.
Choose a topic according to the instructions of your assignment, and make sure it's not too broad or too narrow. For example, you were assigned to write a research paper related to football. Searching "football" alone would be far too broad, and searching for "head injuries sustained by football players at Northside High School" would be far too narrow. The first search would give you thousands of results, and the second would give you few-to-no results. A good example of a topic for this assignment would be "Concussions in the NFL."
Try to turn your chosen topic into a research question. A research question provides a clear focus for your research, guiding what information you should look for. It also allows for exploration and the possibility of unexpected findings, while a statement can limit your research to a specific perspective. Your research should eventually guide you to the answer. Example of a good research question: "What are the psychological impacts of recurrent concussions on NFL players, both during and after their careers?"
Through your research question, you should be able to identify keywords that will help you search for sources related to your topic. Your keywords should be the most important words from your research question. Using the example from earlier, our keywords would be: "psychological impacts", "concussions", and "NFL players." It's a good idea to identify related keywords too, such as "NFL athletes", "mental health effects", "traumatic brain injury", etc.
Use those keywords you identified and plug them into search engines!