Use the following link to create one then submit the URL to your sitemap to Google in your webmaster tools dashboard. http://www.xml-sitemaps.com/
If you have the knowledge of xml than you are able to generate xml site map yourself else there are lots of free tool to generate xml sitemap. As indyonline suggest you the link which is the best tool to generate xml sitemap but only upto 500 pages not more than that.
Check my signature for tutorials on creating XML sitemaps (Google sitemaps) and HTML sitemaps (visual sitemaps)
It's true you should this site and create sitemap for your website and you should have knowledge for XML. it's simple go to google and find it.
Very first visit xml-sitemaps.com/. Than put your url there. change frequency remain - Always last modification--use servers response priority-automatically updated Start Than you will find a message- your sitemap is ready download the XML. Than download it and you will find the sitemap.