Once you have registered as an iPhone developer, you can log in to the iPhone Dev Center and download the iPhone SDK.
At the time of this writing, Apple combined the iPhone SDK and Xcode into a single download. It is possible that in future releases you may need to install Xcode first, and then install the iPhone SDK from a separate installer.
Newer prerelease beta versions of the SDK may be available to those enrolled in the iPhone Developer Program; however, the current stable version will be available even if you choose not to pay to enroll in the program.
The combined download of the Xcode development tools and the iPhone SDK is around 2.5 GB in size. The combined bundle will be downloaded as a disk image file. After it downloads, the image should automatically mount; double-click on the iPhone SDK and Tools package file to install the SDK, as shown in Figure 2-1.
Figure 2-1. Installing the Xcode development tools and the iPhone SDK
The installer will ask you to agree to the terms of the software license agreement before prompting you to install the software. You should install it in the suggested location, and the default installation options will include everything you need to develop applications for the iPhone or iPod touch. However, you’ll need at least 5.9 GB of free space on your disk to install the Xcode developer tools and the iPhone SDK.