The top level domain here is WordPress.com and blogs hosted here are on .wordpress.com sub-domains.
Changing to another .wordpress.com address
If you want to change the .wordpress.com blog address ie. the URL starting with http:// to another .wordpress.com blog address, then you must be logged in as Admin under the exact same username account that registered the blog to do that, and the URL you desire must be available. If so then you can change the URL by using this guide http://en.support.wordpress.com/changing-blog-address/ Note that the change is permanent and irreversible.
Purchasing a domain mapping upgrade
Important Notes Before Upgrading http://en.support.wordpress.com/domains/#important-notes-before-upgrading
In order to map to a domain one must have an underlying .wordpress.com subdomain blog to map from and the domain name you desire must be available for purchase, or you must already own the domain URL.
You must be logged in as Admin http://en.support.wordpress.com/user-roles/#administrator under the exact same username account that registered the blog to access the blog's dashboard at > Store > My Upgrades to purchase a domain name by purchasing an annually renewable domain mapping upgrade for the blog.
See here as you do not own a domain http://en.support.wordpress.com/domains/
After you do the primary blog set up it can take between 24 - 72 hours for domain name propagation to take place throughout the internet. You can view the DNS changes here > http://www.whatsmydns.net/ What's important during that stage is to be patient.