Subscriptions that are in trial state should be activated by adding the card details and from then on the subscription should be charged each month from the day of activation.
Subscriptions in "Trial" state can be changed to "Active" state and card details of the subscription can also be added using ChargeBee Checkout Existing API.
This gives the PCI compliance for the card details entered by the subscriber.
Check out the sample apps & tutorials on how to use the "Checkout Existing" hosted page API
Once the subscription has been changed to the "Active" state, ChargeBee takes care of billing the customer based on their plan.