Dear friend, By default they have sitemap called atom.xml which is stored at following location http://yourbloggername.blogspot.com/atom.xml You can USE the same URL, if you would like to submit your sitemap to Google or Yahoo.
but i will suggest you to upgrade your blog using blogger or wordpress for the full control over css codes than the site map would be more easy to use..
thanks for the information ......... I am having a custom domain in google i was looking for this from last month
frnds i have a problem when i add atom.xml thay accept them but after some time they say there are some errors in this sitemap what is this???????????