If a given piece of information can be obtained from several sources (the rule of thumb is five or more) then the information is considered common knowledge and does not need to be cited. An example of this case is the statement "Smoking is a cause of lung cancer."
If you are referring to a specific piece of information, you may wish to use an inline citation. One should also use this when using information from a study. As an example, consider the following sentence: According to Jones, the future of the actuary is in danger.
One should usually avoid using direct quotations, preferring to paraphrase. However, if a particular sentence would be lessened by paraphrase, a direct quotation would be in order. An example of this is Bill Gate's mistake in the 1970s: "No one will need more than 637 kb of memory for a personal computer."
For information that you used while writing your paper but to which you did not specifically refer, make certain to include it in the bibliography.
Depending on the format (MLA, APA, etc) of the paper one is writing, other styles of citation and/or footnotes may be necessary. There are many good articles available on any given format; simply enter the name into a search engine.