A Sitemap is a file containing a list of the Web pages on your site. It is used to give search
engines information about your site, including URLs that might not otherwise be discovered by
search engines during the normal crawling process.
One Sitemap cannot list more than 50,000 links. If your site has more than 50,000 links, you will
need to create several Sitemaps. It is recommended that you also create a Sitemap index file
listing your other Sitemaps.
Sitemaps can be created manually or you can use a program to automate this process.
There are huge websites provideing a sitemap generator that creates an XML sitemap file based on the way you. You can in in google. Keyword- "Sitemap Generator"