The best start to any research is curiosity and interest. Choose something you are interested in, because you'll be spending time with the subject and going deep into the topic.
Resources to help you begin:
CSL Online Library (scroll down to Writing)
Library Research Guides for Professional Studies
Research Process Tutorials
For the very curious, or driven, STOPPING research is often harder than starting. How do you know when you have gathered enough research?
Usually, you start to see the same experts in the topic referenced.
Check the bibliographies of the articles and books you are reading.
When you feel like you start seeing the same names referenced in the research, you begin to know the major players in a particular subject. At this point, you have done enough.
If you find yourself going off onto the many branches offered by any topic, you need to rein yourself back in, unless you have time to follow a new "limb" of research.