Very simple, go to Google and search "Free Sitemap Generator", and there are quite a few sites that will allow you to type in your URL and it will crawl your site immediatly and create your sitemap, download it save it as an xml file to your sites directory. I use this one http://www.xml-sitemaps.com/
After you download it, submit it the following address; urdomain.com/sitemap.xml Submit it to Google Webmaster Tools...
Must you download it to your own site? or could you use the sitemap creators link for submissions? I ask because I have my site pointed to another directory and can't add anything to it.