How can create a license key for Customize Software using DB

Question

Hello I am working a software firm and I am also a student. I faced a problem, its about my customize software license key. I am developing a POS software for my company .I cannot understand how can i create a license key and i search internet and asked many people but nobody help me.Please if anyone can give me some direction that how can i create it and apply it using database or without database please................ I am now working with SQL express 2005 enviornment, my IDE is Visual Studio 2008 and programming language is C#