Obviously, for mobile development, your studying approach can be:
- Studying mobile in general and focus on develop solutions crossplatforms. There are some tools like PhoneGap that convert your app to all platforms. But, following this approach, you lose some features that are particular for each platform and the code may be not so optmized, according to what I've heard (I never focused my studies on it to be honest).
- Studying Android. Obviously, it is not my focus here to teach you Android, but as a new Apple developer, and for all that I know as someone who already studied Android development in the past, I advise you to focus on iOS, at least for the beginning. iOS development is, in my opinion, easier to learn, and the tools and support for developer are better.
- Studying Apple platforms development. I advise you to start studying Swift. You can develop apps for iOS (and other Apple's platforms) in two languages: Swift or Objective-C. But, Objective-C is older, while Swift is a new language that is easier to learn, and Apple is focusing on it. With Swift, you can develop apps for all Apple's systems (like iOS, OS X, Apple Watch or Apple TV).
There are a lot of content on the internet, but some places that I recomend you to start is through free Udacity courses (there are a few for iOS and Swift). Another great reference is https://www.raywenderlich.com.
Good luck on your journey, and don't give up