Re-using data saves time and money. If a dataset already exists to test your hypothesis, it may save you time and money to re-use it. All research councils encourage the re-use of research data (see, for example, the ESRC Secondary Data Analysis Initiative). Some research councils, including BBSRC (pdf), ESRC and NERC, require information about pre-existing data to be included in your Data Management Plan.
More and more researchers are opening up their data online and you may find data attached to published journal articles or through online data repositories. Not all data can be made publicly available and in some cases restrictions on re-use may be in place.
Re3data.org - a registry of research data repositories - can help you find a research data repository relevant to your academic field.
As with citations of written publications, there are variations in the style and format of data citations. Many journals, data centres and repositories may have their own preferred styles, so you should consult their specific guidance on citing data.
One possible format, recommended by DataCite is: Creator (Publication Year): Title. Publisher. Identifier