One of the most important things you need to answer here is what's your programming background? In case you have never programmed before, then it might be smart to start with
Java as it is an official language for Android, is supported by Android Studio, and
Java has plenty of resources and tutorials to help you learn.
Kotlin is another official Android language and is now Google's preferred language of choice, however, its not as widely used outside of Android Studio.
In case you come from web development, you can go on with
C# you can go with those.
It really depends on your personal experience with programming languages, and what direction you see yourself going down the road. If you only see yourself developing Android apps, then perhaps
Kotlin is the right choice here for the long term.