Create a Pagetemplate (Content-Type:text/xml) in your custom Folder

paste the code:

 <?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="http://www.google.com/schemas/sitemap/0.84"
        xmlns:tal="http://xml.zope.org/namespaces/tal"
        xmlns:metal="http://xml.zope.org/namespaces/metal"
         tal:define="results python:container.portal_catalog(
                             portal_type = ['News Item',' Document', 'Topic'], 
                             review_state=['published'],
                             sort_on='modified' ,sort_order='reverse');
                    dummy python:request.RESPONSE.setHeader('Content-Type', 'text/xml');
                    dummy2 python:request.RESPONSE.setHeader('charset', 'UTF-8');"
         tal:condition="results"
         tal:on-error="nothing" > 
   <url tal:repeat="result results">
       <tal:block tal:define="resultObject result/getObject;">
            <loc tal:content="resultObject/absolute_url">http://www.yoursite.com/</loc>
            <lastmod tal:content="python: DateTime(resultObject.modified()).HTML4()">2005-01-01</lastmod>
       </tal:block>
   </url>
 </urlset>

into it.

This Pagetemplate list all your published contents of the types News Item, Document and Topic. Add all contenttypes or states you want to have listed in the sitemap.

you can add your sitemap to Google Sitemaps at http://www.google.com/webmasters/sitemaps/