You need to put in license
option in your application.
There are different ways to achieve this
1) encrypted license files that contains the expiry dates etc.
2) encrypted license keys held in the registry
3) challenge/response mechanism which require an internet connection
4) probably others.......
Have a look at these: License Key Generation
] or: How can I create a Product Key for my C# App
] or: c# Licensing
They are in C# which can be looked at for starting.
Cryptography 101 for the .NET Framework
Hope this helps.