Once you create a Facebook account, you are assigned your own Facebook profile URL. This is usually just a mix of letters and numbers that is uniquely yours. If you want your profile to be easily remembered or viewed, it is a good idea to change the username.
You can do this by going to ACCOUNT, which can be found in the upper right of the Facebook page. From the dropdown menu, choose Account Settings. It will display a page where you can find Username. Click on that and choose from the options given. Alternatively, you can input your preferred username. Remember, though, that this can only be changed once, so make sure that the username you choose is the one that you really want to use.