Here are a few links to help you out with your APA citations.
Do the easy stuff first. Find out about your theory, the history, the people first.
Then search for the current or new articles about your theory. Remember to use "quotes around the phrase" or do a subject search for the theory.
Once you've got a good pile of the contemporary stuff, you should dig through the literature review section and bibliography to see what articles get cited a lot. Those are your classics.
Take the citations of the classics and find the full-text via the library.