When you join IMDbPro, you'll need to select between the monthly or annual membership plan to ensure you're charged the correct fee when your free trial ends. Billing is automatic when your free trial ends, when your account is still active. You can always switch your membership plan before the end of the free trial on the Membership page.
You can cancel your membership at anytime during the free trial period and your card won't be charged. You'll still have access to IMDbPro until the end of your trial.