Look at broad themes and topics, using resources like textbooks, subject dictionaries and encyclopaedias.
Background reading can also help you to identify key authors and texts (whose bibliographies you can use to generate further ideas).
If you find a really useful article, you could mine that for all sorts of other useful related material.
For example:
Has the author written additional material on the topic?
Does the article have useful keywords or subject terms you can use for further research?
Does the article have references or a bibliography you can use to explore related material?
Does the article link you through to other related material?
It can help to do some brain storming to map out the main topics/concepts/themes/key writers you will be looking at.
Ask yourself questions such as;
Break your research topic down in to a number of smaller sub topics and address those in turn, before bringing everything together to answer your overarching research query.
As you move through your research you may wish to refresh your scoping exercise in order to encompass new areas or to close off particular avenues of research that you considered at the outset.