Should I use singular or plural keywords in google adwords to get more traffic?

When using broad match, use always singular keywords.
Google will count it as singular AND plural.

Dog = Dog & Dogs

Try reading this Adwords guide, it's the best resource for Adwords questions:
http://www.adwordssecrets.net/resource.php?m=adwords-guide

I should say the cell phones is XXX or I should use are in my essay?

Plural Keywords are always the preferred keywords if they have relevancy on the page that they are being applied to.

Looking at specific keyword searches on most search engines will display the plural usage of the keyword, with the singular usage highlighted

Although it is good practice to include both variations in the content of the page and in PPC. Becareful because you might get penalized for keyword stuffing by having all those variations in in meta tags for example exercise, exercises, exericise equipement, fitness and exercise equipment, and exercise and fitness equipment. Think about how your visitors will be thinking. Will someone think of buying jump ropes or a jump rope.

Singular or Plural,use the one as long as it is very relevant to the landing page...

It is a Common Misconception that Plural will give more traffic than Singular keywords..the Latent Semantic Indexing is not as important as itsounds...

Instead use highly related synonyms and related keywords in the adgroup as it will make your adgroup more targeted and bring high quality traffic..