The first step would be adding the site to your webmaster account. To do this, follow this link, and then click on the "+ Add a site..." button. Then Google will verify that the site is yours before allowing you to retrieve any data from the blog. In order to verify the blog, you have to copy the meta tag or HTML fragment provided by Google in to your blog's source code (follow the instructions provided by Google or see the example on that page).
Once your blog has been succesfully verified, go to Dashboard and click Site Configuration > Sitemaps > Submit a Sitemap and then just type "atom.xml" at the end of your blog's URL as shown below.