Should I add a Site Map to my website? I was told that a site map can help me get better rankings in Google. Is it true?

Indirectly maybe. Google can find your pages more easily if you have a sitemap.

off course . It is important to have a site map in the root directory of any site .by that search engine can easily crawl you site

definition:A:sitemap; non-human readable xml map designed to allow easy access by search engineranking:good, intelligence:high; Y/N:veryY
definition:B:sitemap; human readable html map designed to allow visitors to get past a badly laid out nav menuranking:bad, intelligence:none; Y/N:veryN, fix the Navhttp://www.auditmypc.com has a very good sitemap generator of the first type
Google webmaster tools explains how why when where

Should I add a Site Map to my website? I was told that a site map can help me get better rankings in Google. Is it true?

Yes adding a sitemap is a must if you want all of your webpages to be indexed in Google. Google itself provides a Sitemap tool and also remember, you should aim to have a 1:1 ratio of pages to indexed pages in Google or any search engine as a matter of fact.