I wanted to buy apps but it says payment method declined even after giving all details help please..... how to buy apps properly using a visa card

What type of visa card is it, debit or credit ? If it's a debit card then not all seem to still be accepted, and might not be accepted at all in some countries.

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 which is therefore the country on your iTunes account). If it is then you could check with the card issuer to see if it's them that are declining it. Or do you have a different card that you could try ?