Dipping a toe into the conversation...
Academic literature does not exist in a vacuum. When a scholar writes a paper, she is aware of what else has already been written on the topic (or she is if she wants to get published). She reviews what has been done, agrees with some, refutes others, and then adds her new bit to the information stream on that topic. Placing the new bit into the existing conversation is the literature review portion of the current style of academic writing and is why academic articles and books have bibligraphies and footnotes.
Those bibliographies and footnotes form a knowledge matrix which defines what pieces of the conversation are deemed important and which are forgotten in the dust. If no one ever cites your work, then you didn't make so much as a ripple. If hundreds of people cite your work, you had an impact. That impact helps define what constitues "classic" literature.
Following the references in footnotes and bibliographies is a tried and true mechanism for locating significant works on a topic. Knowing how to deconstruct a citation (e.g. figure out what it is, like a book, a book chapter, an article...) and then get your hands on it using the right tools is a fundamental upper-order information literacy skill.
Sometimes you want to go in the opposite direction time-wise. It's useful to know what works were cited in the paper/book you are reading, but has anyone cited the work you are reading? The citations in the bibliography will always be older than the work. If you are looking for newer related material, using a "citation index" will tell you where else that work has itself been cited; it pushes you forward in time to the newer parts of the conversation. It can also tell you whether anyone thought that work was "worthy" of being read and cited in the continuing conversation.
In addition to individual works, periodical publications can be more or less likely to have things from them cited. Something called an "impact factor" can be calculated for a given periodical which can tell you if articles from it are more or less likely to be read and cited. This impact factor changes over time as certain periodicals go into and out of vogue. Nonetheless, there are also classic periodicals as well as classic single works or articles.
Google scholar is the poor-man's citation index. Formal citation indexes exist and are very expensive because of the time consuming detail work it takes to produce them. You may go and use the Citation Indexes at Clarkson if you would like. However, the interconnectedness of hyperlinks and the sophisticated programming the Google has done gives a flavor of how this works. The first piece of information given for one of their results links is the "cited by" information - it is part of how they rank their results and is why often times older rather than newer material rises to the top of the list.
The Google Search
For this class, the google search is best used to find lists of classic works, or to find a known existing work. Searching topically for a psychology topic is less likely to yield the classic material and more likely to give results for newer research or for advocacy sites.
Time sensative Aspects:
Styles of Communication
The popularity of certain types of communication channels changes over time in response to cultural factors. In the most general way, we have gone from learned letters (way back) to "monographs" (books written by one or just a few authors) to articles in scholarly journals to a variety of electronic media. So don't be surprised if your classic works are books (think Freud) rather than articles. What was the preference during the time period you are looking for?
Watch your mouth!
Language changes. Our current "correct" versions of words relating to mental health, mental ability, ethnic and religious groups, etc. changes according to current social, cultural, and behavioral mores. If you are looking for older written material, they would have used the words in use at the time they wrote, not what we would think to use today. So...