Instructions to change your blog URL are found in this support doc: http://en.support.wordpress.com/changing-blog-address/
Please make sure you read the support doc and all instructions and warnings carefully as this process cannot be reversed.
When prompted whether you'd like to keep the old URL or delete it, consider keeping it as deleted URLs can never be recovered: http://en.support.wordpress.com/recycling-blog-names/
Rather, set the old address to private, and then you can either use it as a test blog to try out new themes and widgets, etc., or you can use it as a backup to your regular blog by periodically importing content to it. http://en.support.wordpress.com/settings/privacy-settings/
As part of the process you'll also have the option to change your username to match the new address. Note that if you do this your current username will cease to exist and you won't later be able to go back to it. If you prefer, you can also keep your username what it is and simply change your display name (what will appear next to comments - on the forum it will still be the actual username that appears). Instructions to do that can be found here: http://en.support.wordpress.com/change-your-username/
To automatically redirect visitors from the old address to the new one you'll need to buy the Site Redirect upgrade: http://en.support.wordpress.com/site-redirect/
That's the only part that will cost you money, should you decide to do that. Note that not buying a site redirect will probably lead to a decrease in traffic as people who follow your blog via RSS feed readers and email won't be automatically transferred to the new address.
You can avoid this by leaving the old address live and public and publishing a post stating that you've changed the address. Then provide a link to the new address so they can visit it and follow the new one instead. After a few weeks you can then set the old address to private.