I see three main options for getting into ARKit depending on your experience and goals.
Option 1 — Develop Within The Apple Ecosystem
If you’re an iOS developer with a lot invested into Swift and Cocoa, you can leverage your knowledge for the Apple ecosystem and starting building ARKit apps directly in SceneKit, SpriteKit, or low level Metal.
Option 2 — Build From Unity or Unreal Gaming Frameworks
If you don’t have much experience in iOS development, it may be more advantageous to skip the Apple stack and start directly in a game framework. Unity is a mature platform with lots of built in functionality, support, and documentation. Many people without much programming experience are able to create interactive experiences on these platforms relatively quickly.
Option 3 — Design 3D Graphics
I’d guess a lot of the first wave of ARKit applications will be focused around placing and exploring 3D objects. If this describes your idea, I think working up in the graphics pipeline could take you most of the way there. Using modeling software like 3DS Max, Maya, or Blender, you could design objects and animations. Once you have that, simple placements in either Unity or SceneKit will be pretty straight forward.