Your username can contain uppercase and lowercase letters and numbers. You should avoid Underscores, spaces and symbols! Once you make an account your password will be emailed to the address that you give, so make sure it's valid! Each email address may only be linked to one account at a time. You can change your email, password and other profile information once you have logged in.
Don't forget that by creating a username name you are agreeing to abide by the TOS and EULA