It's a step-by-step process:
Think of a topic.
If it is too broad, narrow it down by brainstorming or reading about it. A general encyclopedia can help with this. Or find an overview article; CQ Researcher is an excellent database for this purpose.
Develop the research question you want to answer or the thesis statement you want to make in your paper.
Use the library catalog to find books.
Use the databases to find articles and other information.
Use websites only if you evaluate their authority, currency, and accuracy.
Evaluate your results as you search.