Subscription to On Demand Apps varies with each content provider. Some On Demand Apps are free while others require a subscription - usually a monthly fee (no contract).
For Apps that require a subscription, launch the App and click on the 'subscribe' or 'sign up' button. Then follow the instructions on the screen within the On Demand App.
They are usually charged per month, without a contract (no commitment and can be canceled at any time except for the Amazon Prime which is valid for 12 months from the day of activation)