Google sitemap is a service that allows you to get many information about what Google think about your page (pageranks, best position searches, errors...). It's easy to use sitemap whan you have your page in a server under your control, but there are some things to take in consideration when using blogger.
Here you have a brief howto:
Sign up Google sitemap
> Add site (your URL)
> Verify your site
> Add a META tag
> Copy META tag
> Sign up blogger
> Template
> Edit HTML
> Paste META tag after
> SAVE TEMPLATE
> Go back to Google sitemap page
> Verify
> Add a sitemap
> Add General Web Sitemap
> Your sitemap URL is {your_blogger_page}/atom.xml
If you need a more detailed howto, you have it here.