Here is how you build and submit a Google Sitemap:
* Decide which pages on your site should be crawled * Determine the Canonical Version of Each Page. * Decide which sitemap format you want to use. * Test using the Sitemaps Testing Tool. * Add the sitemap it to your robots.txt file * Submit the sitemap using the Search Console.
See the documentation here - support.google.com 
# General sitemap guidelines * Use consistent, fully-qualified URLs. * Don't include session IDs from URLs. * Point out translated versions of a URL. * Use Entity Encoding. * Format dates and times using WC3 Datatime format * Break up large sitemaps into a smaller sitemaps. * Use a Sitemaps Index file. * Use recommended canonicalization methods. * Familiarize yourself with our Webmaster Guidelines.
# Sitemap extensions for additional media types
Google supports extended sitemap syntax for the following media types. Use these extensions to describe video files, images, and other hard-to-parse content on your site to improve indexing. * Video * Images * News * Feature-phone-only pages
# Submit your sitemap to Google
After creating your sitemap you need to tell Google where it is so that you are sure that Google crawls it and therefore crawls all the pages that your sitemap indexes.
To do this you need to Submit your sitemap to Google.
# Troubleshooting sitemaps
See the sitemaps troubleshooting guide - support.google.com