The best way?
A designer told me once: Web-Design is not the same as (iOS) App-Design. You haven't the same size of screen, the same kind of action (touch vs mouse, etc). Just adapting a web-site to an iOS App is not recommended. You have to transform it.
Use an iPhone/iPad/iPod Touch. You have to get familiar with the OS (I can't find it, but that's a recommandation from Apple). What's possible, what's not, etc. What does the user expect. Even if there are quite similarities with Android & iOS, Android user won't like app that's designed for iOS-life and iOS user won't like app that's designed for Android-like.
Once you've understood how works Apple Apps (Settings.app, Clock.app, etc.), you may download various apps to understand what can be modified, get ideas, etc. Also, there are plenty of gallery of apps idea, but they aren't always doable.
There are also some guidelines from Apple: https://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/
You can also get some .psd with the "classical" interface for iOS (buttons, NavBars, etc.) on the web.