This document is a quick guide to preparing for iOS Development.
Many resources require an Apple Developer Account. Enrolling for the free account gives you access to the basic resources to start development. You can skip this step at first and simply download Xcode with an Apple ID (an existing one, or a new one you create just for development). You can always return to enroll in the Apple Developer Program once you are ready to release your app (see below).
This program costs $99 a year and is only required if you want to distribute your apps in the App Store and have access to beta iOS releases.
If you have enrolled as in the Developer Program, you can download Xcode from the iOS Dev Center
While you can download Xcode from the Mac App Store, it is limited to the current public release. Developer Accounts can access beta releases of Xcode and iOS, which are available months before the public release. They may also access previous releases, which are valuable for testing very old builds that may no longer compile in the latest Xcode.
Apple announces a new version of Xcode in June, and releases the first beta shortly thereafter. They ship the final release in September.
If it's June, it can be difficult to learn with an Xcode beta, as you may wrestle with bugs in Xcode itself. If it's August, it's safer to use the latest Xcode beta. It's probably stable, and you'll avoid learning techniques that may be deprecated in the latest release.
Do not learn iOS Development on an IDE other than Xcode. Xcode is the only IDE Apple has blessed for iOS development. All documentation is written assuming you are using Xcode. It is more difficult to seek help on forums, as your problem may just be your alternative toolchain.