Why do I need a credit card when I am downloading a free app from the app store on a iPad mini?
I don't know of any way to set up an existing Apple ID for use with the App Store without supplying a credit card (or other payment source) (unless it's an iCloud account, I think -- it's not completely consistent). Don't ask me why this is, it makes no sense to me. There are a couple of ways to set up a paymentless account though:

Create a new Apple ID account as part of the purchase of a free app (see Apple Support article HT2534). You'll need to sign out of your current Apple ID, then start purchasing a free app, and tap Create New Apple ID when asked what ID to use.
This means that your App Store &quot;purchases&quot; will be under a different Apple ID than whatever you have set up under your current ID, but since they're free apps anyway, that's not too big a deal. If you want to purchase non-free apps later, you can always add a credit card (or iTunes gift card or whatever) to your old Apple ID and do those purchases under it, but it may be a bit alloying to have to maintain multiple identities. Note that the new Apple ID will have to be linked to a verified email address (other than the one your current ID is linked to).

Add a credit card to your current Apple ID, &quot;buy&quot; a free app, then change the account's payment type to &quot;none&quot; (see Apple Support article HT1918). Since you never actually buy anything, the card never gets charged. I haven't really experimented with this option, but AIUI you can continue to purchase free apps under the account even after the credit card is removed.