I'm not understanding on how to write one.
Now Alyson, the answer to this is simple. I would first recommend finding out which style of citation your teacher wants you to use. Then, I would recommend asking your teacher for help or a worksheet pertaining to that type of citation. You could also ask your school or public library librarians for aide. If nothing else I would look it up online. Here are some links for two different types of citation.
http://owl.english.purdue.edu/owl/resource/560/01/ (This is for APA)
http://owl.english.purdue.edu/owl/resource/747/01/ (This is for MLA)