First of all, What is a sitemap?A sitemap is a file where you can list the pages of your site to tell Google and other search engines about the content of your website.
To submit a sitemap,
You will need to verify your site with Google webmaster tools. To do this, visit
https://www.google.com/webmasters/tools/home
Sign in to using the same account you use to sign in your Gmail, Blogger etc.
Click on "Add a site..."in the top right corner of your browser, then enter the base URL of your Google Site in the box that pops up, then click"Continue."
Then click "verify"Click on your site URL and then click on the plus-sign next to Site configuration. When it appears,click on the Sitemaps link.
Click on the "Submit a sitemap" button and input the URL of your site XML file. To get your site XML file, you can use an XML file generator.
Google will check the sitemap and begin indexing your site.

OP, I have tried all you wrote but I still couldn't get it. can you kindly help me do it? I will appreciate it.