If possible, select a topic that interests you. If your instructor assigns a topic, make the most of it by finding something about it that interests you. Narrow the topic as much as possible.
Example:
Dental Hygiene - periodontal disease - Gingivitis
But do not narrow your topic too much.
Example:
Dental Hygiene - periodontal disease - Gingivitis- acute ulcerative necrotizing gingivitis - acute ulcerative necrotizing gingivitis in infants - acute ulcerative necrotizing gingivitis in infants in College Station Texas
Having too narrow a topic will severely limit the amount of research you are able to do, while having too broad a topic will require spending all of your time looking at resources.
Explore your topic using a reference resource like an encyclopedia.