adobe photoshop - As a designer, how should I start designing an iOS app?

I am about to design an IOS 8 app for the very first time.

I've been a designer for 5 years now, both on Photoshop and Illustrator, but I never got the change to design an app. I am looking for some guidelines.

My questions are: Which program should I use? Photoshop or Illustrator? I am much more comfortable on Photoshop, but I am afraid of the sizing. Which brings me to my next question, which canvas size to pick, iPhone 4, 5, 6 or 6+? Or maybe a design for each size (same style though)?

Answer

First and foremost you should start at "Designing Great Apps" and "Designing for iOS". If you don't aware of these sources your app certainly will be rejected by Apple.

What to use? Anything you are most familiar with, Illustrator, Photoshop, Pixelmator or any other app you can run, and better - on Retina display.

You should perform a lot of searching and planning ahead of placing any single line on your graphic software - you deal with Apple and should clearly understand what they want from you and what you want from the users.