usually you can setup scheduled task in your hosting control panel to generated sitemap automatically, please check your control panel documentation or contact hosting support for more details. The command line to run generator is:
what I can do to save the sitemap.xml in my root? Set I permission of root tu 0666 ?
You should manually create empty sitemap.xml file in domain root and set the file permissions to 0666 (do not change permissions of root folder itself, only sitemap.xml file).