In order to make iPhone apps, you need an Intel Macintosh computer with the latest OSX operating system, the Apple iPhone developer's kit (it's a free download but it's a large file), and the ability to write computer code in (I believe) a variant of the "C" programming language.

Once you have all of the above in place, to get a basic application functioning is very easy. Whether or not your idea will be easy to implement depends entirely upon exactly what that idea is. Some computer programs are easy to write, others are hard.