Here are some ways to narrow your topic:
Theoretical approach: Limit your topic to a particular approach to the issue. For example, if your topic concerns cloning, examine the theories surrounding of the high rate of failures in animal cloning.
Aspect or sub-area: Consider only one piece of the subject. For example, if your topic is human cloning, investigate government regulation of cloning.
Time: Limit the time span you examine. For example, on a topic in genetics, contrast public attitudes in the 1950's versus the 1990's.
Population group: Limit by age, sex, race, occupation, species or ethnic group. For example, on a topic in genetics, examine specific traits as they affect women over 40 years of age.
Geographical location: A geographic analysis can provide a useful means to examine an issue. For example, if your topic concerns cloning, investigate cloning practices in Europe or the Middle East.
Once you identify your general area of interest, start clarifying your ideas by asking questions. Clear, specific and answerable research question(s) are essential to a successful review. For instance ‘Is family therapy an effective treatment for anorexia nervosa?’ will be a more effective question and produce a more focused set of search results than ‘How do I help people with eating disorders?
Though you want a clearly defined topic at the outset, it is a natural that the topic will alter as you learn more about what has been writtlen about it. However it is important that you retain a clear point of view or theory as you work.