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.
What forum are you running? helps to say what you are running as most forum software and even WordPress has it build into it and you only need to add what you want then upload
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.