There quite a few guideline which Apple insists that your app follow( Human Interface Guideline).
If you have not worked on mobile platform before ,
you should know that
efficient code is a huge, 1.a) priority. due to limited memory & CPU cycles.
Code with badly managed memory etc will be chucked out.
Getting used to Xcode.
If you are
testing on a real device , getting the provisional profiles, installed etc.
These are various facets & there are number of things that could go wrong.
You will get several Hello World programs on the internet, but learn the basics of Iphone programming
& Objective C first. Else you would be confusing yourself... a lot.
Cheers !!!