Your topic is normally not finalized before your first visit to the library. Often the focus will change as you do research and become better informed.
Many people have trouble taking a broad topic and narrowing it down to something more manageable.
A particular age group, occupation, ethnic group, men, women, etc. For example, if you are interested in writing about depression, you might focus on depression in children, or depression in the elderly. You may find it helpful to state your topic as a question. A specific country, province, city, rural vs. urban environment, physical environment, etc. You may want to focus on the suggested causes of the problem or issue you are researching.