function button

I can't even imaging how to start with this so I'll ask - I would like to make a button so the user can turn on/off a function - in my case a custom screen saver. I've seen the theEndless has a "function" button or what looks like it in the Instant Watch app. I know i'll have to get into saving the choice in the box-persistence wise but i'll worry about that later, just want to work out the function on/off thing now - Know its some kind of if/else statement, if someone can direct me where to start that would be great.

Thanks