If possible, find where this information originally was taken from, most professors don't like wikipedia for citation.
In general, if you don't have the information, it's alright to omit it. In this case, there's no author, so just leave it out.
"2011 Yemeni Protests." Wikipedia, the Free Encyclopedia. Web. 16 Apr. 2011. <http://en.wikipedia.org/wiki/2011_Yemeni_protests>.
remember to indent the second line and on.