I guess it also depends on how much (other) programming you've done in the past too.
I'd recommend starting on this website:
Cocoa Dev Central
Read through all the tutorials to get an understanding. It'll help to re-visit these tutorials about once a week as they'll make more sense and you'll pick up things you missed.
Next, you should signup to the iPhone developer program (it's free) and download the SDK (Mac only):
iPhone Dev Center
From here you can download sample code to achieve 90% of the things you'll need in an application.
Follow the "Creating an iPhone Application" getting started document - This is a detailed guide to creating a basic "Hello World" example.
Finally, once you have the SDK installed and you've started using Xcode just click "Help -> Documentation" and from here you can search all of the iPhone APIs etc.
Basically, get comfortable with object-C, Cocoa and then play with the sample code.
If you like learning from books then the best one at the moment would probably be:
Cocoa Programming for Mac OS X, Third Edition
by Aaron Hillegass