paying for a app.

It uses Google Wallet. There's a list of ways to pay here: Payment Methods - Google Checkout Buyer Help

Unfortunately it looks like the only options are credit card and direct carrier billing, meaning it would go on your cell phone bill. (I don't think all carriers support this.)

 

If your carrier does not support carrier billing then yes, credit card is your only option.

Google will automatically setup a Google wallet account when you make your first purchase. Wallet works similar to Amazon's One Click purchasing.

If you don't want you credit card in Google Wallet, after your purchase, you can use a PC to log into your Google Wallet account and remove the credit card.

 

You can also go into Walmart or any grocery store and get a pre-paid credit card.