How to add license functionality to a VB.NET application

I am using VB.NET for coding and SQL Server 2005 for database.

My app is ready for release. Just missing things before I can live.

I want to implement the license concept in my application .

My license file can be text or xml file . In which there should be some kind of encrypted question that will be used to validate the application.

Please suggest how can I implement this functionality in my application

Thank.

Best wishes, Solution