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 cancelled at any time except for the Amazon Prime which also offers a 12 months contract from the day of activation.