To purchase in-app content you first need to add a valid payment method to your Google Account. You can add several payment methods (Card, DCB, Paypal) to one Google account.
Add a payment method
  1. Open the Google Play Store app 
  2. Touch Menu > Account > Add payment method
  3. Follow the onscreen instructions
  4. The new payment method will be added to your Google account
For more information about available payment options go see this Help Center article.

Remove a payment method
  1. Open the Google Play Store app 
  2. Touch Menu  > Account
  3. Under Payment methods, touch More payment settings
  4. If prompted, sign in to payments.google.com
  5. Touch the payment method you want to remove
  6. Touch Remove > Yes, remove it
The payment method will be removed from your Google account.

Edit a payment method
  1. Open the Google Play Store app 
  2. Touch Menu  > Account
  3. Under Payment methods, touch "More payment settings"
  4. Sign in to your payments.google.com account
  5. Touch the payment method you want to edit
  6. Touch "Edit"
  7. Make your updates and tap "Save"
Change your payment method while making a purchase
  1. Open the game 
  2. Go to the shop
  3. Touch the package you want to purchase
  4. Tap on "Payment methods"
  5. Choose from one of your existing payment methods or add a new one
  6. Complete your purchase
Note: if you add a payment method while making a purchase, it will be saved in your account.