PayPal doesn't require you to create a username, but you should link one e-mail address to your PayPal account. You'll also need to set up a secure password.
The e-mail address you use is up to you. Just make sure it's an address you use often and can easily log into to get notifications about when your money comes through. You will give PayPal a lot of sensitive information, so make sure your password is secure. Use a combination of upper and lower case letters as well as numbers and special characters (like #, !, @, etc.). A random string of numbers, letters, and special characters (i.e., 13b%E56s$T89!) is generally better than a recognizable word (i.e., HaPPy123!).