This is a terrific question, and learning how to assess research materials now will stand you in good stead throughout your college career and beyond. The term "scholarly" simply means that the work is better researched and more precisely written than a general knowledge work. It is not a precise term, and there is no magic formula for determining if a written work is scholarly. The guidelines to determine whether a work (article, book, website) is scholarly are not universal, but here are some general principles from which to work.
The content of any material must be applicable to the subject you are researching and should be written in scholarly language. (This means the work should be written for research purposes rather than as an overview of a subject which is written for the lay person.) Be sure, however, that the writing is not so obtuse or technical that you will not be able to use it effectively; it may still be a scholarly work, but it will not be useful to you if it is undecipherable.
The author or editor of the work should be someone who carries some authority in the field about which he is writing. This can generally be ascertained by looking at the author's biography in or on the book, but it can also be done by a quick Internet search.
The content (the actual research findings) of the article or book should be documented, either in a bibliography or in footnotes (which can be found at the bottom of a page, the end of a chapter, or the back of a book). This documentation has the added benefit of giving you more sources, as well; if you read something which is particularly helpful, check the original source for more information.
When possible, use the primary (original) source rather than a secondary source. For example, if an article quotes a study, find and use the original study rather than assuming the article is accurately presenting the information in its entirely.
Books which have been published by major universities are generally considered scholarly; however, you must still examine the source itself for the elements listed above.
The Internet should probably the last resort for locating scholarly work because it is so difficult to ascertain authorship and documentation, though you can certainly find some useful research materials. Once you have determined the major researchers or studies for your topic, you will have a better sense of what might be both useful and scholarly. The Georgetown link below provides some specific ways to evaluate websites.
As a general rule, if something does not look or sound scholarly, it probably is not. It is always best to check to see if the English department at your university has set guidelines and, if in doubt, always check with your professor.