The point of the site map is to help Google index your website. The easiest way to create one is to have your site use a content management system like WordPress, Joomla, Drupal or be built on a framework install and activate a plugin designed for the content management system If you've chosen not to use one of the CMS then you'll probably just need to query your own database for the pages or write by hand. Until we know more about your site we can't tell you more.
If you have your own made websites / mixed technology look into desktop sitemappers like GSiteCrawler and A1 Sitemap Generator (search google for more)