Citations/references/footnotes are the way scholars indicate what resources they have used and give credit to the work/ideas of other scholars.  Publishers and instructors will require that your citations be in a particular format--and there are many.  Fortunately, a few will suffice for much of your work.  Here are some of the more popular ones:

You will find that these works also offer many other wonderful suggestions to help you in your writing.

Searchworks and many other databases you will use allow you to get a simple citation by using a "CiteThis" feature or gather more information by using a Send to/Export feature to send records to bibliographic management tools such as Refworks or EndNote.  You can also use resources such as Zotero to gather information including citations.  For more on these, see the guide to Bibliography Management.