Great question and not something I will be able to answer fully, as a lot of if will depend on how you like to learn, but I believe this answer will provide you with a few hits and tips to get started.
First, let's look at the platform you want to build on, that's going to influence your research considerably. If you want to build an iOS application, I would recommend that you look into Swift. If you're looking to build Android apps, I would recommend that you put your time into learning Java or Kotlin. If you want to build an application that works on both with the same source code, you could look at Cordova, Flutter. Take a look at some of the documentation for some of these, watch a few tutorials and see which you think would be right for your project.
Right now I am learning during my spare time using a combination of MIT app Inventor 2 and Android Studio.
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