How to disable billing payment

You can download free apps, but you may need to have a valid payment method on file, even if the app you want to download is free. 

Free apps has also In-app pruchase.

See -> Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card.

If your card is not accepted as a paymenf method, you need to contact your bank.

If you don't have a credit card or don't want to use a credit card, you can also use your Apple Account balance as payment method.

See -> Add money to your Apple Account balance

Add money to your Apple Account balance - Apple Support

Where to buy a physical Apple Gift Card or App Store & iTunes gift card: actually, you can buy an Apple gift card at many places, at an Apple Store or an Apple Authorized Reseller in your region.

See also this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support 

If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.