You'll need to learn the basics of programming (in dependent of the language used)
You'll need to learn C and Objective C
You'll need to learn how to use Xcode
You'll need to learn the Cocoa touch frameworks.
For learning programming and C/Objective C, I'd recommend "Objective C, the Big Nerd Ranch Guide". It teaches you programming from the beginning, using first C, then Objective C, and teaching you a little Xcode along the way. It's well-written and pretty easy to follow.
If you read that book from cover to cover, and do all the exercises, you'll have a decent foundation in Objective C.
Then you'll need a good book on iOS development, or perhaps the Stanford video series, or maybe both.