If you want to develop apps for the iPhone, iPad, Apple Watch, Mac or Apple TV, you will need to enroll in Apple's Developer Program. You need an Apple ID and you will have to provide some basic personal information. Other than that, it's a $99 per year subscription.
Once you are a member you will have access to all of the latest resources, including development tools, beta software, advanced app capabilities, beta testing and app analytic tools. And if you do develop an app, you will be able to distribute it via the App Store.