Sitemap is a listing of all the webpages of your site and if you submit your site and the sitemap at Google Webmaster Central, it will be additional view of the site as per google. It is not mandatory that you submit sitemap to get your pages indexed.
Google Says
A Sitemap provides an additional view into your site (just as your home page and HTML site map do). This program does not replace our normal methods of crawling the web. Google still searches and indexes your sites the same way it has done in the past whether or not you use this program. Sites are nevër penalized for using this service.
Its very simple, you can go to Google Webmaster Central and submit it. There are many tools available to create sitemaps in XML format. Even for Wordpress we can create sitemaps with ease.
Some tools are
www.auditmypc.com/free-sitemap-generator.asp
www.xml-sitemaps.com/
If you are using any CMS like Joomla, drupal or wordpress, then you can use the available plugins with which you can generate sitemaps on the fly. Make sure that you verify your site at Google Webmaster central and submit a sitemap for your site. This can produce better results. Google does not say that your page will or will not be indexed if you submit a sitemap or do not submit a sitemap. It will include your site map for the next crawl.
