Learn the language, and read Apples developer policy very, very carefully. Every app goes through a testing phase and is required to not interfere with certain functions of the phone. Apple claims that those types of apps that perform a task better than their own may “cause confusion for the user”. I’ve seen a number of apps that made it to the app store and were later removed, or were denied altogether.
I have developed an app for mac os and I can tell you that Big Nerd Ranch really helped me.
I was completely new to Objective-C but coming from any object-oriented language, you’ll be able to pick it up very quickly. Also, keep in mind you will have to buy a $99/yr developer license from apple to put your app on the app store. Furthermore, be wary of sandboxing (entitlements) your app, it’s a pain in the arse if done in the end. Gradually check that functionality you’re adding step by step can work in a sandboxed environment.