Some changes in the info plist file to tell the project what's the app delegate for the iPhone and which is the one for the iPad
Make a view for your controller which makes use of the larger screen size
The simplest thing to do, IMHO, is that you create a new universal project, and copy your existing code to it. Doing so will automatically do the three steps for you. However, you'll have to make your own customizations of course.
Don't mention it. One more thing though. iPhone applications work on iPad devices, but they use only a small portion of the screen (same size as the iPhone screen), so technically, iPhon apps will work on iPad apps, but they won't make use of the iPad screen size