iOS Dev Center http://developer.apple.com/devcenter/ios/index.action
This is the central portal for development on Apple platform i.e. iOS.
iPhone Programming In particular to start the iphone apps development, here is an excellent introduction to the iphone programming.
The first thing you need to do is to download the Xcode, which is the key development tool in iphone programming. You also need to have a Mac workstation or notebook using Intel processor to have Xcode running.
Within this tutorial, you need also learn Objective C, which is the main programming language on iOS platform.
You may follow the iphone programming tutorial to build your first iphone app.
There are various development links on Internet for iphone development
1. Many iphone programming tutorials
2. Discussion forum on iphone programming
3. Another popular development forum
Set up development environment
Developing In Eclipse with Android Development Tools