iOS is the operating system for Apple's iPhone, Touch and iPad (that name still makes me cringe). Want to learn how to build apps for these devices? Take a class. Udemy offers a straight-forward one.
iPhone app development is also now offered to high school students! MakeGamesWith.us offers an online tutorial for those with some object-oriented programming skills (found in many AP computer science classes). The site teaches Objective-C.
Note: a Mac is required.