The primary condition is that you should have a firm grip on the Objective-C programming language that is an elite form of the C programming language.
Now what you need to do is to register yourself at the Apple Developer program. Do not mistake it for a free membership, as it is a premium registration. Apple offers two kinds of membership for an individual it charges USD99 while for an enterprise membership, the fee has been declared to USD299. You will also find free development kits but do not be misguided as Apple shows very little or no support for them.
Start working on your app using the above mentioned language to ensure the support of Apple. Be certain about the requirements of iPhone to avoid as much trouble as possible. Your app should be bugs free and support the iOS and if any bug arises in future, Apple will require you to fix that on immediate basis.
You should become a member of the Apple Developer Connection and after that you can give in your application to Apple for a review that will eventually lead to acceptance, if they found it suitable.
Do not forget to update the contents of your app to meet the current criteria of security and compatibility with updated softwares.