Billing keeps declining my payment method what should I do

What are you trying to use as your payment method, credit or debit card ? If it's a debit card that might be why, not all seem to still be accepted by iTunes, and they might no longer be accepted at all in some countries. Have you got another card that you could try ? Or do you get the 'none' option so that you can remove it : Change or remove your Apple ID payment information in the iTunes Store - Apple Support ?

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).