A sitemap is a file that describes the layout of your site including all categories, pages, and posts. It includes data such as the last time the page was changed. This file is used by search engines to help them navigate and process pages on your site.
There are a number of plugins that can handle creating the sitemap.xml file for you. However, there is one that is regularly updated and gets good ratings: Google Sitemap Generator.
Log in to your WordPress Admin panel and click on "Plugins". Click on the "Add New" button and search for "Google XML Sitemaps". You'll want the one that is by the author Arne Brachhold. You can also download the plug directly:
Once you have installed it, click "Activate" then configure it for your site.
Configuring the Plugin
Once the plugin is active, hover over the Settings menu and click on XML Sitemap.
On this page, there are a number of options that you can change to suit your site. Most of the defaults are good for a new site, however, you might want to change the post priority.
If your site is going to be a social site where you expect comments and conversations the best setting for this is comment count. This raises the priority of highly active posts.
However, if your site is a business site or a normal blog with few comments then select the "Do not use automatic priority" option. All posts will have the same priority as set in the general configuration.