To use the Google Webmaster Tools for your blog, you must first add your blog and allow it to be self verified. This way you tell google, that this ‘site’ is indeed yours.
For a blogger account, the help is given here. If you choose the meta tag verification method for your blogspot blogs, here are the steps to get it verified.
Open your dashboard by visiting blogger.com. Proceed to Settings > Layout > Edit HTML > EDIT Template.
In the code provided copy the <meta> content you picked up from the google’s webmaster tools page. After saving the template, proceed to webmaster page and click on verfiy.
Also to update the sitemap url, proceed to http://yourblog.com/robots.txt copy the sitemap url and update it under Webmaster’s sitemap section.