Citing sources is a key part of your research; it documents what sources you have used in researchin and writing your paper, and gives credit to the author's work that you have used. It also gives information to identify and retrieve the cited sources.
A word of advice -- keep track of your information sources as soon as you begin the research process. Fortunately, many of the electronic databases you will be using offer the option to download, print or email the citations to the material you find but it is always more difficult to backtrack to find the one that got away.
There are many citation styles available and the preferred style varies between disciplines. (Prof. Richard Ascough's website contains a section on Footnote & Bibliography Format for an overview and subject-specific examples.)