Sitemap consists of lists of URLs of a site with metadata (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) about each URL, which helps informing search engines about site pages that are available for crawling, so that search engines can more intelligently crawl the site.

Lets consider we have following urls and priority of the urls are as below -

priority 1(highest): http://ganeshhs.com/

priority 2: http://ganeshhs.com/clients

priority 3: http://www.ganeshhs.com/services

priority 4(least): http://ganeshhs.com/contact-us

As of above http://ganeshhs.com/ , takes highest priority. Lets write a sitemap (sitemap.xml) considering the above criteria -

<?xml version="1.0" encoding="UTF-8"?><urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

<url>

<loc>http://www.ganeshhs.com/</loc>

<lastmod>2007-05-01</lastmod>

<changefreq>weekly</changefreq>

<priority>1.0</priority>

</url>

<url>

<loc>http://www.ganeshhs.com/clients</loc>

<lastmod>2007-05-01</lastmod>

<changefreq>daily</changefreq>

<priority>0.8</priority>

</url>

<url>

<loc>http://www.ganeshhs.com/services</loc>

<changefreq>monthly</changefreq>

<priority>0.8</priority>

</url>

<url>

<loc>http://www.ganeshhs.com/contact-us</loc>

<changefreq>monthly</changefreq>

<priority>0.6</priority>

</url>

</urlset>

Explaination of xml tags -
UTF-8 format : Sitemap xml file encoding should be UTF-8 format

Escape code : All data values should be entity escaped (for example, character & should be escaped as &, character ” escaped as &quote;)

<urlset> : Sitemap starts with this tag , which is a required parent tag, encloses the <url> tags

<url> : is a required tag, which encloses info of each url of the site.

<loc> : is a required tag, which is enclosed within <url> tag, consists of the URL. This URL must begin with the protocol (such as http) and end with a trailing slash, if your web server requires it. This value must be less than 2,048 characters.

<lastmod> : is optional tag. The date of last modification of the file. use YYYY-MM-DD, is the date format.

<changefreq>: is optional tag. How frequently the page is likely to change.
Valid values are always, hourly, daily, weekly, monthly, yearly, never

<priority> : is optional tag. The default priority of a page is 0.5. Valid values range from 0.0 to 1.0.

Once you write the sitemap, ping to search engines. Below are the ways where website ganeshhs.com sitemap is pinged to search engines.

http://www.google.com/ping?sitemap=http%3A%2F%2Fganeshhs.com%2Fsitemap.xml

http://submissions.ask.com/ping?sitemap=http%3A%2F%2Fganeshhs.com%2Fsitemap.xml

http://search.yahooapis.com/SiteExplorerService/V1/ping?sitemap=http%3A%2F%2Fganeshhs.com%2Fsitemap.xml

Reference link -
http://www.sitemaps.org/