iPhone 101
Code Tips
Resources
More
This tutorial was written for Xcode 3.2; it's outdated now. For a more up-to-date tutorial, check out Apple's App Development Tutorial to learn how to write iOS apps. There are also lots of other excellent books and online tutorials you can learn from.
If you'd like to find out when the updated version of this class is ready, follow me on twitter: Follow @idev101
Tools
You're going to need a Mac. You can't develop Mac/iPhone apps on anything but a Mac. Any modern (Intel-based) Mac will do - even the Mac Mini!
Eventually you're also going to need an iPhone or iPod Touch for testing your apps. If you don't already have one, there's no immediate rush; the developer tools allow you to test your programs in the "iPhone Simulator", a Mac application that simulates the iPhone.
Sign up for a developer account
It's free; you can register and download the developer tools without having to pay anything. When you get far enough along that you want to start testing apps on your phone and selling apps on the app store, you can become a registered developer for $99/year.
Download the SDK
After you've registered, log into http://developer.apple.com/iphone/ and download the SDK (that's "Software Development Kit"). You'll need a fast connection for this; the iPhone SDK is over 2 gigabytes in size.
Once you've downloaded and installed the SDK, there'll be a new "Developer" folder in the top level of your hard drive. Look inside Developer -> Applications and find Xcode:
Drag the Xcode icon into your dock.