Song of Solomon by Toni Morrison is a widely respected novel by a Nobel Prize–winning novelist and has been the subject of a substantial amounts of literary critical analysis. One solid starting point for your research would be:
Jan Furman. Toni Morrison's Song of Solomon: A Casebook. Oxford University Press, 2003.
More generally, to find literary criticism of the sort your professors require, namely that published in peer-reviewed journals, you should search academic and related databases.
First, rather than use the regular version of Google, you can use GoogleScholar (https://scholar.google.com/), a site that restricts searches to scholarly sources. Next, you should go to the your university library's website and access two other databases:
The MLA International Bibliography: a comprehensive database with citations for literary scholarship.
JStor: A full-text database including over a hundred years of journal articles across all academic disciplines (with 27 pages of search results on your topic).
As you search these databases, enter a group of keywords with binary operators or "+" signs to narrow your search results to the specific work, e.g. by using as search strings "+morrison +song +solomon" or "morrison AND song AND solomon."