Where do I start?
If you already have an Apple ID (either from iTunes/MobileMe/.Mac/etc.), then the first place to start would be to join the iPhone DevCenter. The basic (free) access will allow you to download the SDK (Software Development Kit), look at documentation, download example code, and test your applications in the iPhone Simulator (a Mac application that emulates the iPhone OS).
You will not be able to test and debug your application on your actual iPhone or iPod touch hardware, however. For that you will need to purchase a paid development membership.
Once you attempt to sign in to the DevCenter with your Apple ID, you will be presented with some terms and conditions that you will need to accept. Once you accept them, you will be taken to the main DevCenter -- this is the place where you will be able to download the SDK and access some other important information about iPhone development.
Read on to learn more about iPhone development.
Should I become a paid member?
Apple offers two levels of paid memberships for the iPhone developer program. The Standard program is $99, while the Enterprise program is $299. What's the difference?
The standard program gives you the ability to create both free and commercial applications for distribution on the iTunes App Store. The Standard program also gives you full access to the iPhone DevCenter, the ability to test your applications on either an iPhone or iPod touch, access to search and submission rights in the Apple Developer Forums, and distribution of your applications via the Ad-hoc method.
If you are looking to develop applications for use within a particular company (meaning you will not be distributing via the iTunes App Store), then you might take a look at the Enterprise program. This program will give you the same level of access as the Standard program, except you trade off iTunes App Store access for Internal ("In-house") Distribution -- which will allow you the ability to distribute your apps via e-mail, download link, etc.
Both forms of the iPhone dev program give you access to code-level technical support for your applications. This means that you can send Apple your project (Xcode project) via e-mail and have them help you with errors that you might run into.
For these posts, we will be focusing mainly on the Standard Development Program; if you will be joining the Enterprise Program, I highly suggest taking a look at the documentation available on the DevCenter as well as in the Apple Developer Forums.
You can find more information about the different developer memberships available on Apple's website. I would suggest playing around with the SDK before you join any of the iPhone development programs -- this will allow you some time to warm up to coding on the iPhone. Purchasing a membership only takes a couple of weeks to set up.
Look for my next iPhone Dev 101 post in which I'll discuss the iPhone SDK, and the applications that comprise it. If you want me to cover a specific topic on development for the iPhone, feel free to let me know in the comments.