Do you have an iPad or an Android device?
For an Android device: To buy things, you'll need to set up a credit card on your account, or buy a Google Play gift card and redeem it in the Play Store on your device. To add a credit card, open the Play Store, tap the "hamburger menu" (usually top left), tap settings then My Account. Select to add a payment method. To redeem a Google Play gift card, select Redeem on the hamburger menu instead of My Account.