Quite often you will be expected to use "scholarly" or "peer-reviewed" sources. How can you tell whether a source is scholarly? Look for these indicators.
As you search, keep track of the most promising sources and then look at them closely, asking yourself these questions.
When you aren't an expert, it may seem daunting to evaluate the work others have published, but a book's table of contents or an article's opening paragraphs will help establish relevance. The language it is written in will help you decide if it is scholarly enough and yet not too highly technical for your purposes. Information given about the author might help you decide how much an authority he or she is. For Web sources, follow links to information "about this site" or to an author's home page, or shorten the URL to everything up to the first slash to see what its parent page looks like. When in doubt, check with your instructor.
Compare: In addition to looking at the quality of individual sources, compare them so that you can see where there are differences and conflicts. Even if you are heading toward a particular conclusion, you want to discuss alternative perspectives so your reader gets the big picture.
Corroborate: If you feel as if you're going out on a limb, try to corroborate the information you want to use in another source. A reference librarian can help you do that.
Sometimes a particular word or phrase seems to unlock your search magically. As you explore your topic, keep an eye out for key words and phrases that the experts use. If you're having trouble, check with your instructor or a reference librarian.
As you decide which sources to use, remember that your choice of sources reflects on you as a writer. If you bring experts along to help you tell your story, your reader will be more impressed than if you bring along opinionated, rude sources that make a lot of noise but aren't well behaved.
When you invite sources to support your argument, it's a little like bringning expert witnesses to a courtroom. You want them to impress your readers.