What is your topic? What kind of a project is it?
How much support do you need? Do you need scholarly materials, or will general information do? Who is writing about your topic? For what purpose(s)? This can make a difference to what you look for, and where. For most academic classes, some of your sources should be of scholarly quality-- that is, articles from peer-reviewed journals, proceedings from important conferences, and the like. The level or complexity of information required often rises with the class level-- that is, most of the time you'll need more and better sources for a senior class than for a first year class, and more if higher amounts of credit are offered for a class.
Some topics are well covered in books, or you can gain a beginning understanding via an entry in a good encyclopedia, or even Wikipedia (it keeps up well with popular culture, for example). Others are better covered in newspapers, magazines, or journal articles; videos and podcasts may be useful for some topics.
Who is considered an expert may depend on what the topic is! Being well-known (notoriety or celebrity) doesn't mean that a person is an expert. Or they may be an expert on one thing and not another; relatively few people are experts in several fields, and anyone can have an opinion.
Are you looking for scholars or practitioners, authors whose credentials are in their years of experience, a combination of those factors, or something else?
The more you know about a topic, the easier it is to evaluate what you need for a particular project- up to a point. Experience counts; having a feel for the field of interest makes a difference, too. This is one reason professors sometimes suggest topics; sometimes they suggest a topic that they want to know more about, too.
If you are giving a panel presentation, you may not need as much substantial supporting materials for yourself as you might for a capstone project or thesis, but you will need to coordinate content with the other panelists. A simple topic may need less support than a complex one, but a complicated topic may be more interesting.
How much time do you have?
How long/how involved does the paper or presentation need to be?
What kind(s) of information do you need-- a single fact, a series of facts that build on each other? a simple analysis, an in-depth discussion, or more?
When did you start working on it? When is it due?
Some information is easier to find quickly in certain places. Specific facts such as boiling points of substances are easier to find in handbooks, or resources like Knovel. Some information is proprietary, and can't be had without paying for it; sometimes we have sources that contain it because of our paid subscriptions.
Are specific types or numbers of sources required? Articles from peer-reviewed or scholarly journals are often required; they are normally considered more reputable than magazines or some other publications, such as blogs, trade magazines, or newspapers. Many of the databases will allow you to limit your search to peer-reviewed or scholarly journals (see the box for a definition of peer review).
There are exceptions, but most people who are considered authorities:
Peer review is the process by which articles or other works are critiqued before they are published. Authors send articles to an editor, who decides whether the work should be forwarded to reviewers for the journal. The most stringent form is anonymous or blind review, where neither the author nor the reviewers know whose work is being examined by whom. This helps reduce bias.
Reviewers are usually well-published researchers and experts. The reviewers return the articles to the editor with remarks and recommendations-- usually publish 'as is' (rare), publish if edited or changed in specific ways, or don't publish. Editors most often go with the recommendation of the majority of the reviewers.
The process is intended to improve the content of studies published-- more eyes on a project, and one's reputation on the line with peers, tends to improve the quality of what's submitted and published. There are cases where it hasn't worked, and critics of the cycle, but it is the best system that has been developed to this point.