Anytime you use a direct quote, whether it's an entire paragraph or a short phrase, cite it. Set off the language with quotations and include either an in-text citation or a footnote. It's not enough to write something like "Dr. Smith of the University of Maryland says that children need at least eight hours of sleep." If the phrase "children need at least eight hours of sleep" is taken verbatim from your source, then you need to include it in quotes.
Whenever you paraphrase information, you must also cite the reference. In the statement "Dr. Smith of the University of Maryland says that children need at least eight hours of sleep," you could summarize "children need at least eight hours of sleep" by saying something like "It's best that children get a good night's rest and get a minimum of eight hours of sleep." Since the idea is coming directly from your source but put into your own words, you don't need a quote but you do need to cite the information.
In general, when you use facts or data in your paper, you must cite them. Even if you know a statistic off the top of your head because you previously memorized it, the information probably needs a reference point. For example, if you use a fact like one in three women will be sexually assaulted in her lifetime, then you must cite the source. However, facts considered common knowledge, such as dogs and cats are mammals, do not need citations.
When you're not quoting, summarizing or paraphrasing something directly, you do not need a citation. If something is considered a generally accepted fact, using this in your paper without a citation is not plagiarism. Note, however, you must attribute ideas that come from people, research projects and studies to the appropriate sources. You also don't need to cite information when you are using opinion statements or making an argument for a thesis -- as long as you're not using anyone's ideas but your own. A rule of thumb is to consider whether most people would know the fact or information; if they would, then it's common knowledge.