Start by figuring out why your app doesn't work on the iPad already. Most apps developed for the iPhone will work fine on an iPad (in compatibility mode) with no modification; if yours doesn't, you must be doing something to prevent it. Are you relying on some hardware feature? Making unfounded assumptions about the device you're running on? How does your app fail when run on an iPad?
Once you've figured out why it doesn't work, you'll be much closer than you are now to fixing the problem.