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
Open the Google Play Store app
Tap on the icon with three horizontal lines > Payment methods
Under "Add payment method", select the payment method you want to add
Follow the instructions and the new payment method will be added to your Google account
For more information about available payment options go see this Help Center article. Edit/Remove a payment method
Open the Google Play Store app
Tap on the icon with three horizontal lines > Payment methods > More payment settings
If asked, sign in to pay.google.com
Under the payment method to be edited or removed, tap Edit or Remove and then follow the instructions.
Change your payment method while making a purchase
Click on an in-game item you would like to purchase
Under the price, tap on the arrow pointing right
Choose from one of your existing payment methods or add a new one
Complete your purchase
Note: if you add a payment method while making a purchase, it will be saved in your account.