It's really site map.xml you want (if your submitting to google).
Your root folder is where you would have installed / uploaded your site in the first place.
It's normally in public_html or httpdocs folder, or sometimes literally the first place you see when you log into your FTP account.
If you use Wordpress its where you'll see index.php, WP-config.php, wp-content and so on.
You'll know if you done it correctly as you'll be able to access the site map by browsing to yoursite.com/sitemap.xml in your Internet browser.