Hi! My credit card has expired and app store asks for its details even for free apps. Whats the solution please.

You will have to add a payment method to the account. iTunes uses that for identification purposes. You can try logging into the computer and trying to select None, but that will not always work, especially if you have not made a purchase, or made one in a long time. You may need to purchase an iTunes gift card to put a balance on your account.