Before you begin searching, try to develop a question that your research will answer. As an example, if your chosen topic is childhood obesity, your question might be "What do studies show about school-based childhood obesity programs?" or "What does the literature say about exercise and childhood obesity?" Once you have some background information on your topic, you will find more specific questions. For example, are school-based obesity prevention programs with an internet compenent more effective than those without?
It will be easier to narrow your search if you define a specific patient population. As an example, if you are working on sucide prevention, possible populations would be "adolescents enrolled in school" or "elderly commuity residents."