When you first register your username is always your email address.
Once you have registered and have your secure password, then you can login, input your username and password.
After successfully logging in, simply click on the "Your Profile" link in the top left margin. The "Update Your Profile" page will display with your current username in the "New Username" field.
Simply, delete your current username and type in a new username and hit the "submit" button.
PLEASE NOTE: YOU ARE ONLY ALLOWED TO CHANGE EITHER YOUR USERNAME OR PASSWORD IN ONE STEP. SHOULD YOU WANT TO CHANGE BOTH, YOU MUST DO IT IN TWO STEPS. FOR EXAMPLE: FIRST, CHANGE YOUR USERNAME, HIT SUBMIT. THEN GO BACK TO THE "UPDATE PROFILE" PAGE AND CHANGE YOUR PASSWORD.