When you make a new page it automatically generate a url for your page, Something like below
The time when you got 25 plus likes on your page, you can have option to choose a easy username for your page. Something like below
You can only choose a username for once. As facebook did not allow for now to change the username again. So be careful while choosing a username for your page as you will not have any second chance.
To change the user name of your page you can either go to directly at this url http://faceboo.com/username and then select your page of which you want to change the username.
2nd option is go to your page then click on Edit button on the right corner of upper side of your page and then choose option update info.