Citation question... unsure of which format my instructor is using and how to handle several authors..?

Question

I have a paper due tomorrow and it's all done, however I'm unsure how to cite my last journal article in the work cited section--the article has twelve authors. My instructor didn't mention which citation style we are using, he instead just gave us the formula and his examples only go as far as three authors, all of which are listed (only "et al." when cited in text, not in work cited).

Do you think I need to write out all authors in the works cited, or just the first three?

If it helps the format goes:
Lastname,_FI.MI.,_Year._Article title._Journal Title,_vol#(issue#):pgs
Second and third authors are FI.MI. Lastname...

Answer 1

For examples of how to write it open link
http://psychology.about.com/od/apastyle/p/intextcitations.htm
................APA format......................
APA format establishes a number of clear rules for how to cite works using author information. In-text citations vary based on the number of authors the source is attributed to. The following guidelines can help you prepare in-text citations in proper APA format.
No Author:

While most articles, books, and other reference materials will include an author attribution, some sources lack information on authorship. How to you cite these resources in APA format? In-text citations should use a short article title enclosed in parentheses and the date. When article titles are long, simply use the first word or two of the title.
.................One Author: ..................
When a book, article, or other source lists only one author, simply provide the author’s last name followed by the publication date.
................Two Authors:..................
When a source lists two authors, your in-text citations should provide the last names of both authors and the publication date.
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........Example for 3 -5 authors.........
For example: …results indicated a strong positive correlation between the two variables (Robsen, Hutchkins, Ru, & Selanis, 1989).

or Robsen, Hutchkins, Ru, & Selanis (1989) found a strong positive correlation between the two variables.
............Three to Five Authors:.............
Proper APA format for sources with three to five authors requires listing the last names of all authors the first time you cite the source as well as the publication date.
Subsequent citations should list only the last name of the first author along with the publication date.
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..............Six or More Authors: ..........
Listing the last name of the first author as well as the publication date should cite sources with more than six authors.
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......... The OWL at Purdue...........
http://owl.english.purdue.edu/owl/resource/557/01/
MLA Formatting and Style Guide
All Sections in MLA Formatting and Style Guide:

General Format
In-Text Citations: The Basics
In-Text Citations: Author-Page Style
Formatting Quotations
Footnotes and Endnotes
Works Cited Page: Basic Format
Works Cited Page: Books
Works Cited: Periodicals
Works Cited: Electronic Sources
Works Cited: Other Non-Print Sources
Additional Resources

Answer 2

probably MLA format....with MLA when you have more than four authors you usually name just the first and then put et.al or you could name all 12