Registration process for shareware

Your program must work in 2 states. Trial version offers only limited functionality, after registering program must switch to state with full functionality. Registration consist from:
Application side
1. GUI for one time entering some input from customers.
2. Algorithm for save this data somewhere.
3. Algorithm for read registration data in start application time.
4. Algorithm which checks if registration data are correct in start application time.
5. Global state variable if is app in trial or full mode
6. Conditions for limiting some functionality in trial version
How to write registration algorithm for shareware?

Web promotion side
1. Download page from application (trial version)
2. Buy page where you offer registration of application
3. Algorithm which generate registration key from buyer input on pages for payment processing, after payment

When you write algorithm for registration key generating, it is important to choose right inputs for this function. Inputs must customer enter while he will pay for registration. Usually it is a customer e-mail, or name. And it is good to find such algorithm which it is possible to write in programming language for application (Java, C++ and etc.) and also for HTTP request, that means in PHP for example.

Next possibility is using external libraries. A lot of companies for payment processing offer this kind of tools on their pages.