Like most services, you need to be registered and have payment methods set up before you can make payments.

As a first time user, you can actually complete your purchase at the point of purchase. There's no pre-registration required and through texting and a phone call you can add your payment information and notify the restaurant of your payment.

Ah, I didn't realize that you could add payment info from your phone. I'm not sure how safe I'd feel about that.