Sitemap Generator is one of the best Sitemap extension for Joomla, download here.

1. Install the extension.
2. Go to Components -> Sitemap Generator in the backend and use the Generate your sitemap button to start the generation process.
3. The sitemap will be saved as sitemap.xml in your Joomla root directory. Be aware that an existing sitemap.xml file would be overwritten without asking.
4. Use the "Show the sitemap" button to download your sitemap and check if the generated sitemap is complete.

Note: The Sitemap Generator service allows you to create a sitemap with up to 500 URLs for free.