This service is free and easy to use. You should create a sitemap file in XML format then submit it to Google Sitemaps and update your site map when change your site.
A sample Sitemap that contains just one URL and uses all optional tags is shown below. The optional tags are in italics.
<?xml version=”1.0″ encoding=”UTF-8″?>
lastmod: The date of last modification of the file.
changefreq: How frequently the page is likely to change.
priority: you set the priority you want Google to place on that page in your site. You can prioritise your pages: 0.0 being the least, 1.0 being the highest, 0.5 is in the middle. This is ONLY relative to your site. It will not affect your rankings.
Google has provided a complete help to create sitemap XML here https://www.google.com/webmasters/sitemaps.
But an easy way is using free sitemap generators. There are many free sitemap generators available on web. Just enter your URL then generate your sitemap and copy the code into Notepad and save as sitemap.xml.
At last upload it to the root directory of your site and submitting to Google sitemaps then every time change your site go to Google Sitemap and click on the resubmit button.