You can go to www.google.com/webmasters and hit the sitemap link there to register. You then need to follow the instructions on how to add the site map to your site and register it with Google. If you use software like Wordpress then you can download a plugin that will automatically generate the sitemap for you. I know this was no where near a comprehensive post from me, but let us know if you need any more details or get stuck anywhere.
Another useful link if you are looking for a sitemap generator is: http://sitemap.contentdesk.net/sitemap.htm
Read the knowledge about sitemap here www.sitemaps.org. Now it's a standard accepted by google, msn and yahoo.