The first thing you need to know is that Apple’s official software development platform for the iPhone works only on OS X so if you are a PC-only household you may be out of luck. Fortunately for me, my daughter has a MacBook, which she’ll share with me if I’m extra nice to her.
Now for the requirements: you’ll want to have software release 2.2 loaded on your iPhone and OS X version 10.5.5 (or later) on your Mac. The iPhone SDK takes roughly 4.4 GB so you’ll need at least that much free space on one of your local hard drives.
So now you’re ready to download the iPhone SDK (software development kit), which can be done from this site: http://developer.apple.com/iphone/sdk1/. You’ll need an Apple ID, which you can create via the “Create Apple ID” link at the above URL.
If and when you get serious about distributing apps to real customers via the iTunes app store, you’ll need to apply to the iPhone Developer Program, which will set you back $99 (one time charge, not per app). The good news is you can “try before you buy”, i.e. download the tools, write and and test some apps, play to your heart’s content, before you commit to the $99 enrollment fee.
I’ll have more to say about the development tools, the IDE, the language, the API, etc. but for now I just wanted to share some quick guidance about getting started. The good news is that if you have a Mac with some available disk space, it’s easy and free to set yourself up as an iPhone developer.