Depends on what you want to make. If you plan to make utility applications, learn how to use the iOS and Android SDKs (Xamarin will allow you to make a single app that can compile for both). If you plan to make games, learn how to use a game engine like Unity3D or Unreal Engine 4 (or make your own).

Once you know what route you plan to take, the languages you need to learn will be obvious. iOS prefers Swift and Android prefers Java or C++ (C++ is a lot better for resource heavy apps, like games). (If you use Xamarin, you can use C# for both). For game engines, it depends on what the engine developers chose. Unity3D mainly uses C# and Unreal mainly uses C++ and a visual scripting system called blueprint.

My biggest tip to you, READ DOCUMENTATION. Tutorials will only take you so far. Once you are proficient in object oriented programming, having direct access to any information you need is infinitely better than any tutorial. If you don't believe me now, you will once you get deeper into development.