There are a number of online Modern Language Association citation generators that can assist you in referencing material that you utilize from the review site Yelp. You will need to know the author of the post, the date that he/she posted it, and the date you accessed the article.
A simple Google search of "online mla generators" will point you in the right direction. I created a fictional citation using the web site Citation Machine, the results of which I placed at the bottom of these instructions. Once you have entered the site, click on 'website' and type www.yelp.com. From there, the citation machine will ask you to fill in the relevant information and then an MLA citation will be generated for you. Copy and paste that citation to your works cited page.
Fictional reference I generated:
Smith, John. "Yelp." Yelp. 12 Feb. 2012. Web. 18 Nov. 2015. <http://www.yelp.com>.