The easy answer to "What makes something scholarly?" is "something written by scholars, for scholars." However, the reality is more complicated than that.
For journals, an article that goes through a "peer review" process can be considered scholarly. Many databases, like Academic OneFile, will tell you whether an article is peer-reviewed. You can also take a look at a journal's editorial page to find out if it is peer-reviewed.
For books, look for key indicators like whether the author is affiliated with a university/college or whether the book is published by an academic press.
Scholary works (especially articles) can be easy to identify. Look for some of the following characteristics:
For additional information, check out NCSU's Anatomy of a Scholarly Article.
If you are not sure whether something is scholarly, ask a librarian or check with your professor.