To get a custom Facebook URL eg. www.facebook.com/my-url simply request your choice of URL from: http://www.facebook.com/username/
All verified Facebook users are eligible to claim usernames and this requires that you have a verified account, which can be done via SMS when you request your custom URL.
If your choice of URL is not available the system lets you search for other options. Take care when confirming your choice or URL as it cannot be changed at a later date.
For more information see http://www.facebook.com/help?page=897
