Blogspot is nice enough to automatially makde one. Check in the the robots.txt file for the location of the sitemap for the site. Then take the URL and ping the search engines
I found with my blogspot site all I had to do was to register with Google Webmaster Tools, verify the website (it tells you how to do this) and then add the sitemap as atom.xml (that's literally all you have to add when it asks for the sitemap address, so in effect you will be telling it that it is located in nameofyoursite/atom.xml). As the previous commentator said, Blogspot creates this sitemap (atom.xml) automatically.