What's your current experience level? Do you know about how to compile iOS apps, how to work with view controllers, how to separate your data model from your display logic through model view controller etc? How about how to use something like Core Data?
If not, you'll need to start by learning Cocoa: have a look at this FAQ. I'm sure other people will be along soon to suggest books that may be helpful.
If you do know cocoa already, the you should be breaking the problem down into smaller parts. Nobody's going to release a calorie tracking API. But if you start by considering what data you need to store, how the user will input it, what calculations you'll have to make etc, then it should lead you on to more specific questions that are easier to answer