How to create a sitemap.xml file to help web crawling

I would like to create a sitemap.xml file for our website.

We have the homepage and 300  article pages under  homepage.

I read from other posts ,we have to create robots.txt file

we have to create a sitemap.xml file .

It would be of great help if someone explians the procedure on how to create these files under what nodes and the logic to create the sitemap.xml file.

Any help on this will be appreciated .

Thanks

Reyansh