You can register your own www.yoursite.com domain name through your website by navigating to the 'Edit Site' tab -> 'Domain & Email', and click on 'I want a domain name from [HOST.NAME]'. Note that only paid sites may attach a domain name.
Once your domain is registered, it will take approximately 24 to 72 hours before it resolves to your website. If the domain still does not show your website after 72 hours, please submit a ticket to support by clicking on 'Help' at the top right-hand side when logged into your site.
TIP: If you created manual links to your own pages before you attached a domain name, you will want to update any links that show your [HOST.NAME].com address. Note that you can create links to your own pages without including any domain name.