In order to upload app to App Store, you must first create a developer account. Fill up the form with the required information and pay the fees.
Note that the payment is higher for iOS apps than it is for Android apps. It is also recurring annually instead of a one-time fee. Therefore, you must make sure that the app will at least generate more value than what you are paying just to keep it online.
You should also make sure that the app has been tested extensively so that it can pass the examination of the App Store team. It should work flawlessly with no obvious bugs. It must be functional enough to merit inclusion in the platform.
It should not have any content that might be deemed offensive. The included screenshots should also be demonstrative of the user experience.