Do a little brainstorming and write down everything you can think of, no matter how trivial it may seem. The purpose is to get you thinking about the topic. For example, Prisons are where criminals go. Some innocent people get sent to jail. Prisoners have little outside contact.
Prisons are overcrowded.
You could keep going. Now you have some more ideas to work with, but you still aren't finished defining the topic. Use appropriate research tools to retrieve good resources You are still trying to define the topic. Browse through the list of records that come up, looking for a book that is especially interesting to you. Identify subject headings.