Hello there,
When you set up the subscription, you aren't billed. You're simply setting up the payment method. If you decide in the first 30 days you don't want to play the game, you can just unsubscribe and never be charged again. The first 30 days is lumped into the price of the game, but you still need to set up a valid subscription.
This process is set up like this for a variety of reasons, including controlling gold farmers and people using stolen credit cards.
You can set up your subscription with a valid credit/debit card or you can buy game cards.
Hope that helps! Good luck!
Danicia