If customers can use your app for free during a trial period, you can design your app to exclude or limit some features during the trial period. You can also enable features, such as banners or watermarks, that are shown only during the trial, before a customer buys your app. Let's look at how to add this to your app.
For step by step guide, visit: http://msdn.microsoft.com/en-US/library/windows/apps/hh694065
You can learn how to:
.... and have a happy New Year! ,)