Take your research question or argument and refine it into some key research terms. It's a great time to narrow your search a bit.
So if my argument is: humanity has had a significant impact on global warming.
My key terms would be: human* and impact and global warming
An asterisk * will pick up various endings of a root word. In this case human* will search human, humanity, and humans
To narrow, you could break down specifics of global warming such as carbon emissions or methane gases; you could focus on specific solutions such as changes in transportation, energy use or diet. Or focus on related negative effects such as food insecurity.