I have an iPhone application, now i want to convert that application to a universal application which runs on all the devices iPhone/iPod/iPad. So, where to start, what thin
I've done that recently, it's actually much simpler than it would seem. I recommend you read the iPad programming guide from Apple, it's about 100 pages in PDF (and you don't need to read all of it). Basically, you need to:
That's of course if you don't want to redesign the UI for iPad, which you'll probably want to do in the end (e.g. use split views, popup dialogs and various modals, and do less full screen view transitions). The UI that you'll get by going through this steps won't feel 100% iPad-y, but it will work, and will look much better than an iPhone-only app zoomed in, so it's a good start.