How and from where can I learn iOS development quickly?

You can learn iOS development quickly by using these kind of stuff:-

1) Book: Programming in Objective c by stephan Kochan

2) Tutorials: http://www.appcoda.com

               htttp://www.raywinderlich.com

3) Docs, samples, videos: https://developer.apple.com/library/ios/navigation/index.html

This AppCoda course is great if you're coming from any programming language http://www.appcoda.com/ios-programming-course/

I think the apple documenation is the best place.

https://developer.apple.com/library/ios/navigation/#section=Resource%20Types&topic=Getting%20Started 

developer.apple.com