Attention: Sitemaps moved in Yoast as of version 7: read here: https://yoast.com/yoast-seo-7-0/
Learn how easy it can be to add an XML Sitemap to your WordPress website using the Yoast SEO Plugin.
Before you do this you also need to register with Google Search Console and verify your website: https://www.youtube.com/watch?v=mkFeQ1ZDcY4
You can download the Yoast SEO Plugin here: https://wordpress.org/plugins/wordpress-seo/
Learn more about WordPress and SEO at Mad Lemmings: http://madlemmings.com