STEP 1:

For Joomla Sites, use XMAP to auto create the site map.

Click "New" and ad at least the main menu.

Edit the site map to increase the priority of the home page.

STEP 2:

Insert this into the .htaccess file to create a link to the sitemap.xml...

## SITE MAP XML
RewriteCond %{REQUEST_URI} ^/sitemap.xml
RewriteRule .* /index.php?option=com_xmap&id=1&view=xml

More info here...

http://joomla.vargas.co.cr/en/documentation/34-xmap-2/how-to/108-xmap-sitemap-as-sitemapxml

STEP 3:

Submit a Sitemap using robots.txt

You can tell Google and other search engines about your Sitemap by adding the following line to your robots.txt file (updating the sample URL with the complete path to your own Sitemap):

Sitemap: http://example.com/sitemap.xml

STEP 4:

Submit into Google Webmaster Tools