WordPress is the most widely used CMS in the world. It powers millions of websites. It started as a blogging platform and has grown into a sophisticated tool for creating websites large and small. WordPress was written and is maintained by a group of enthusiasts. As such it is freely available to download and use. Thousands of themes and plugins have been created for WordPress over the years which have made many of our lives a lot easier. Some of the themes and many of the plugins are freely available. There also exist a large number of extensive plugins and professional themes with a ton of features which require you to pay for them.
If you don't have your own hosting yet, you can always use WordPress' own hosting solution which is great and of course a dedicated hosting fine-tuned for Wordpress sites. Altough the Wordpress software is free of charge, you will have to pay a hosting fee to Wordpres if you decide to host your site with them. You can also buy and register your domain with them.