Start by looking at broad themes and topics, looking at resources like textbooks, subject dictionaries and encyclopaedias that examine larger background concepts, before narrowing your search to look for specific research and articles in your area of study.
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.
An important part of the planning process is identifying and outlining the topic areas of your research subject. It can help to do some brain storming to map out the main topics/concepts you will be looking at.
Mind mapping can be a useful way of capturing these concepts, themes and sub themes - but use whatever technique works best for you.
Ask yourself questions to help you start thinking around your topic, 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 this exercise in order to encompass new areas you discover.