Hello I'm trying to fill my billing information but it keeps telling me that they are declining my payment info and to try another one although I checked many times

What are you trying to use as your payment method, credit card, debit card ... ? If it's a debit card then they no longer seem to be accepted in all countries, and even where they are it seems that not all work.

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 and your iTunes account are. If it is then you could check with the card issuer to see if it's them that are declining it, and if not then try contacting iTunes Support (these are user-to-user forums) and see if they know why it's being declined (but you might not be able to use a debit card) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

Are iTunes gift cards available in your country (they are not available in all and they are country-specific, they can only be used in their country of issue) ? If they are then you could potentially use them as your payment method.