It's prior knowledge and then doing small projects or demos that enable you to learn the things you don't know. Take you time. Going from a desktop or web programming area to mobile is a bigger leap than you think. Especially because it sounds like it will involve learning the elegant and interesting objective-c.

For mobile programming I recommend watching the Google I/O and WWDC videos. They are verbose but it is the real deal.

Otherwise I like to do small tech demos or do tutorials from books.

Learn the IDEs better right away too. option-click stuff in Xcode and learn whatever Android Studio is supposed to do(I kid).