Things to learn before starting to build an android app?

I'm seriously looking into building an android app, a fairly complex one. I was wondering what are the things/programming languages I need to learn before I embark on this journey. I learned java language, I'm beginning on my tutorial of Android studio, is there anything else that I can learn that will make this a smooth experience for me? Thank you.

Also, I'll be needing to transfer this app to iOS. Is there a best known method to go about it? I don't wanna have to rewrite the entire app in C++ all over again.