Even if you’re using a sitemap generator that automatically submits your sitemap to Google, Yahoo, and Bing, it’s more than worthwhile to manually submit your sitemap to Google using Google Webmaster Tools. Though automatic submission does make Google aware of your sitemap, manual submission will allow Google to verify your ownership of the site, allow you verify that your sitemap is being indexed, and will give you access to Google’s many diagnostic tools to help you keep your site in top shape.
Check out this how-to video to see, step-by-step, how to submit your sitemap to Google:

