Hi gjoisa,
There are several ways to do a trial version of your program.
You can for example disable some functions (for example Save). To do so, simply comment out the Sub that does this. Or even easier, remove (or hide) the buttons that command the actions.
You can also limit the use of the program to a certain duration (10 minutes, for example).
To do this, you can use the Tick count, and after a certain level, you simply close the application, with an explanation.
It depends of your application. You need the user to be able to use it, but not too much. Or too long.