Just as important as knowing how to cite your sources is knowing when to cite your sources.
This guide from Princeton University is an excellent overview.
(Princeton University. Academic Integrity at Princeton University, 2011. http://www.princeton.edu/pr/pub/integrity/pages/cite/)
Academic Integrity Policy
Students of the university must conduct themselves in accordance with the highest standards of academic honesty and integrity. Failure to maintain academic integrity will not be tolerated. The following definitions are provided for understanding and clarity.
Definitions of Plagiarism, Cheating, and Academic Dishonesty
Student plagiarism is the presentation of the writing or thinking of another as the student’s own. In written or oral work a student may make fair use of quotations, ideas, images, etc., that appear in others’ work only if the student gives appropriate credit to the original authors, thinkers, owners, or creators of that work. This includes material found on the internet and in electronic databases.
Cheating entails the use of unauthorized or prohibited aids in accomplishing assigned academic tasks. Obtaining unauthorized help on examinations, using prohibited notes on closed-note examinations, and depending on others for the writing of essays or the creation of other assigned work are all forms of cheating.
Academic dishonesty may also include other acts intended to misrepresent the authorship of academic work or to undermine the integrity of the classroom or of grades assigned for academic work. Deliberate acts threatening the integrity of library materials or the smooth operation of laboratories are among possible acts of academic dishonesty.
See the Undergraduate Bulletin or specific Graduate School policies for more information.
This video from the Lehman College Library explains how to paraphrase without plagiarizing.