Mobile ( Ios ) Tutorial / How to for a "all screen" action

Hi Everyone,
I’m used to create app for Desktop with “wel defined” button to press , different pages displayed …etc.
I just started new project with mobile ( hence my post in getting started rather that the Target/Ios category as I guess it is a basic thing ) and I was wondering how to handle the fact of a “all screen” action. By this, I mean the following :
I have defined in a previous step an icon that I want to drop anywhere on the screen. How should I proceed with this ? is there any tutorial or exemple(s) for me to follow ?
What are the differences between then desktop / mobile ( now Ios and in close future the Android …)
Thanks in advance