Q: i cant register my debit card to apple id...........when i am tring to register........it shows your payment method was delclined please  enter a valid payment method what should i do???

If it's a debit card then I don't think that they are still accepted as a valid payment method in all countries - from this page :

You may be able to use other payment types in your country, like debit and Maestro cards.

which implies that they are not accepted in all countries, and there have been a number of posts about them being declined.

Is the card then is it registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account, it was issued by a bank in your country and you are currently in that country ? 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 and see if they know why it's being declined : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Account Management.

Or do you have a credit card that you could use instead ?