Even with a 30-day free trial, you will still need to add a form of payment since you are signing up for a membership. There is no charge during the free trial, but if you don't cancel your membership during the trial period, you will be billed at the end of the trial for the following month.
Please note: You may receive an initial authorization charge (usually $1 USD) on your account to verify the payment method. This will be refunded shortly after the payment has been confirmed.