you should upload created sitemap.xml file to your site and submit it to Google here: http://www.google.com/webmasters/sitemaps/siteoverview
Basically, steps are described on our homepage: http://www.xml-sitemaps.com/
1. Enter your full website URL and some optional parameters in the form below.
2. Press 'Start' button and wait until the site is completely crawled (the progress will be indicated)
3. You will see the generated sitemap details page, including number of pages, broken links list, XML file content and link to a compressed sitemap. Download the sitemap file using this link and put it into the "public_html/" folder of your site.
4. Go to your Google Webmaster account and add your sitemap URL.