Sitemaps are useful for both people, as well as search engine robots (spiders) in indexing all of your website for their search engines. To make this as easy as possible, especially Google, install the WordPress plugin called Google XML Sitemaps.
Once you’ve installed and activated the plugin, you can look over the settings by clicking in the WordPress Dashboard under Settings -> XML Sitemap. You should be able to leave the default settings, but go through them just in case. Then notice at the top of the XML Sitemap settings page it says that the sitemap has not been generated yet. Generate it now, and you’ll see that not only does it generate it for you, it notifies all of the popular search engines (Google, Bing, Ask.com etc) of your new, clean sitemap!
Whenever you change your website around, create a new page, or add several new posts, create an updated sitemap!