Login name - this is not case-sensitive. There is currently no facility to change this once it's set, so pick one you can live with. You can use letters, numbers, spaces, plus the 2 extra characters '-' and '_'.
Password - Passwords should be at least 6 letters, with a mixture of capitals, lowercase and numbers. Confirm new password - type it again Please note, this site only stores passwords in encrypted form. That means that if you forget it, there is no way of retrieving it or working out what it might have been. Pick one that you'll remember. And give a working email address in case you forget anyway ...
Email address - This is not obligatory, but I strongly recommend that you give one (and subsequently keep it up to date). It will not be passed on to anyone else, published in any way or used for any purpose except to enable the site or its administrator(s) to contact you if it ever becomes necessary. If you forget your password, for example, the button on the login form will use it to mail you a new one; if you haven't given one you lose access to them for ever.
Real name - if you provide this it will be shown on the "view tune" pages as the owner of the tune in question (otherwise your login name will be used). I'd suggest that it's useful, and polite, for users to be able to see who you are when they're browsing your tunes, but it's up to you.
"About" - anything you'd like to tell us about yourself. This is visible via the link from your published "Real name" (as supplied above) on the "view tune" page. This can contain HTML markup if you are familiar with that - ie, links to websites, mailto: links to give a contact email if you'd like people to be able to contact you in connection with your tunes; etc, etc.