Here are some helpful tutorials for you

Getting started with Unity

https://unity.com/de/how-to/beginner-video-game-resources

Careful! Please do not try to make each and every tutorial! You should understand at least

a) the Unity user interface

b) how to work with Game Objects and C# - Scripts in Unity

c) how to transform (translate and rotate) a game object in a 3D game world (so, it might be a good idea to start with some tutorials on 3D games and not the 2d platformers...)

Check, which AR framework will work on your phone and decide, which features you will need

For Android, the current AR framework is ARCore. Here, you can find a list with supported devices: 

https://developers.google.com/ar/devices?hl=en

For Apple, the framework is called ARkit. Here, check whether your device is supporting at least iOS 11.

?What if my Android device is too old? → There is a proprietary library by PTC called Vuforia that also supports older android devices and some features (object tracking, area tracking) not supported by ARCore yet (https://library.vuforia.com/getting-started/getting-started-vuforia-engine-unity)

Setting up ARFoundation (Cross-Plattform AR Library for Unity)

https://learn.unity.com/tutorial/setting-up-ar-foundation#5fe2be51edbc2a1f5e698730

Interaction methods for mobile AR 

Using the gaze (a focus point) as a selection tool
 

Image Targets as marker for Screen-independent 3D interaction