Once you have a basic idea of what your topic is, you'll need to get an idea of the big picture. What are the major issues involved in your topic? What are the terms associated with it?
Reference sources are a great way to begin your search. They'll show you which sources experts recommend and give you ideas for specific areas of the topic you may want to research. These are often broad and general, so it's best to start here and narrow your topic down as you progress.