What is a Google sitemap? A Google sitemap is an xml file containing a list of the URLs on your site.
The easiest way to create one is to use an online sitemap generator. If you try Googling the search term 'online sitemap generator' you will get a substantial list of sites offering this service for free.
However I have found that there are some issues involved in generating a sitemap for a Joomla site using this kind of service. Joomla sites often have a complex menu structure, with important content pages located several clicks away from the home page. Many popular sitemap generators seem to have problems adequately crawling a Joomla site. For example, I found that one popular and widely advertised sitemap generator only managed to index seven pages from this site!
One sitemap generator that I can recommend for Joomla sites is http://www.sitemaps-builder.com/ .
It can be slow, but I have found that it does do a good job of indexing pages located several levels within the menu structure.
If you want a sitemap for your Joomla site, the important thing is to try several, and then to check that they are actually indexing your pages correctly. You can do this quite easily by opening up the generated sitemap.xml file in a text editor.
A Google sitemap takes the general form:-
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://www.mysite.com/</loc>
<priority>0.5</priority>
<lastmod>2009-07-30T08:19:27+00:00</lastmod>
<changefreq>monthly</changefreq>
</url>
<url>
<loc>http://www.mysite.com/example-page.html</loc>
<priority>0.5</priority>
<lastmod>2009-07-30T08:19:37+00:00</lastmod>
<changefreq>monthly</changefreq>
</url>
<url>
<loc>http://www.mysite.com/example-page-2.html</loc>
<priority>0.5</priority>
<lastmod>2009-07-30T08:19:37+00:00</lastmod>
<changefreq>monthly</changefreq>
</url>
.
.
.
</urlset>
The page addresses are contained within the <loc> tags, a quick check through them will show whether your pages are being properly indexed.
It is important to remember though that a sitemap is merely advisory, and is not normally essential. A few years ago it was the case that Google often only looked at a few pages on a site, and including a sitemap could make a substantial difference to your Google ranking, by making it easier to crawl. These days the Google robot does a very good job of crawling even very complex sites, and can index most URLs, even those containing complex query strings, so it is doubtful whether a sitemap makes a large difference. However even making a small difference with your Google ranking is worthwhile, so it is worth doing if you can.
Once you have generated a satisfactory sitemap, you need to upload it to your site, to the www root folder of your site. The you need to notify Google. To do so, type the following into the address bar of you web browser.
http://www.google.com/webmasters/tools/ping?sitemap=http://www.yoursite.com/sitemap.xml
replacing yoursite.com with the URL of your site. You should receive a notification message saying that the notification has been received.