>So for an API could I make one myself or should I just purchase one?
You should read more about APIs and what they're used for. https://stackoverflow.com/questions/7440379/what-exactly-is-the-meaning-of-an-api
You would build the logic, the REST API, in your application to allow other services to interact with it. You don't necessarily need to do it depending on your applications needs.
You can use Objective-C to write your IOS app or check out the Swift language. Apple has tons of free resources for learning it.
So the first thing I did is read through all of Apple's Objective-C tutorial here. I then went through Apple's UI/App Development Tutorial (in Objective-C, removed from Apple's website) here. I'm very new to tweak development, so these YouTube tutorial videos really helped (here). For tweak development, I wouldn't have gotten anywhere without those YouTube tutorials I linked above, so if you're looking for a place to start, try that.
Nope. I hear the same from college graduates, one telling me "Objective-C is never going away".
EDIT: In fact, from Apple's own documentation:
>Objective-C is the primary programming language you use when writing software for OS X and iOS.