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.