Sitemap of any website helps web crawlers or robots like Google bot to identify each page on the website and your pages start appearing on Google Search faster than ever.
In order to create a sitemap for your blogspot blog, use following URL -
http://<yourblog>.blogspot.com/atom.xml?redirect=false&start-index=1&max-results=500
In order to submit your sitemap to Google Webmaster tool -
1.) Open the webmaster tool dashboard.
2.) On the dashboard, you will see a list of sites submitted by you on Google Webmaster Tool.
3.) Click "View Details" link in-front of any of your sites for which you would like to add sitemap.
4.) On the next page, you will see a button called "submit a sitemap". Click this button, which opens a new page where you will see a button called "Add/test sitemap" on the right hand side corner. Click this button.
5.) It opens a pop-up with a text field. You can enter following text in that text field.
6.) Click "Submit Sitemap" button.
Now your sitemap has been created & submitted to Google Webmaster tools.