Put your xml file in the root directory and put the URL to it anywhere in your robots.txt file. Your URL should be http://yoursite.com/sitemap.xml
Google will eventually crawl it, or you can ping Google so they get to it quicker.
I can't post links yet - but, do a quick Google search for a sitemap generator. Let it crawl your site and give you a download link for your new sitemap.xml file. Drop that file in the root directory of your site. Google and other search engines will eventually crawl it.