Q: how to ..

Do you mean an iTunes gift card code ? That field is optional, you don't have to fill it in (do you have an iTunes gift card ?) - and as iTunes gift cards aren't available in all countries you may not be able to fill it in anyway

iTunes can ask you your Apple ID email and password. If you don't remember it, follow this instruction If you forgot your Apple ID - Apple Support

You only have one billing address on an iTunes account.

Or do you mean email addresses ? If you do then you only have to have one email address (the primary email address), the rescue email address is optional - though it would make use to have a rescue email address on your account (you can create a free email address via http://gmail.com or http://hotmail.com) : About your Apple ID email addresses - Apple Support

Having a rescue email address is optional but recommended. If you ever forget your security questions, you can use your rescue email address to reset them. If you don't have a rescue email address, you'll need to contact Apple Support for help. We’ll also send any account security-related notifications to your rescue email address.