First I would like to say that I mean no disrespect about what I am about to say. But code project has a great search function in the upper right hand corner. I just typed in "licensing" and I got these great articles.
Cryptography 101 for the .NET Framework[
^]
License Key Generation[
^]
May these can help you? Take a look and let me know.
One last thing I would like to mention it's hard helping someone if they give you little information. Wish you all the best.
Keep on coding! :)