You would need either a credit or debit card. This card must have one of the major processors (AmEX, Discover, Mastercard, or VISA). This is needed not just for the one time purchase, but also a continuing subscription for as long as you have the number. This card will also be used for adding money to the funds pool which I will recommend at least $10USD in order to make sure you would always have the money in case you go over your minutes, or make an international call.
If you are opposed to credit cards, then you would use your debit card as long as it has one of the 4 logos mentioned above. This is required to pay for service. Many banks will include one of these logos on your card. This should be easy enough to find on your card.