A Google Sitemap is similar to regular site maps, but is a machine
readable file (XML) that Google uses to see which of your pages are more
important than others and how often they are updated. Google then uses
this information to decide how frequently your pages should be indexed.
You can write your own Google Sitemaps, the XML is fairly straight forward, but why bother when you can use a tool like this to generate one for you:
Once your Google Sitemap has been generated, you will need to upload it to the root directory of your site. You can do this with a dedicated FTP client such as Cyberduck, Fetch, or Transmit. Alternatively, you can use the Upload Extra Resources action. Apply the action to your main index page, and specify that the Sitemap should be uploaded "With Page’s HTML File".
We recommend signing up to the Google Webmaster service http://www.google.com/webmasters/tools.
This service not only allows you to submit Google Sitemaps, it also provides
information about your Google search ranking, what position your site appears
in the search engine results page (SERP) for sepcific search terms and also the keywords that Google sees on your pages. All of these things
will help you make your site even more findable.