The most important thing you should learn to develop mobile app is programming language. Given below is the list of some programming languages you should learn.
Java is a object oriented programming language that can run on any hardware or software platform. Java can run in two different ways - in a browser window or in a virtual machine. A virtual machine does not need a website. You can create different types of apps using Java.
Python is a high level programming language that is used in app development, software development, web development etc. It is easy to use and also readable. It is also very compatible. Any type of mobile app can be created using Python.
PHP is the abbreviation of Hypertext Preprocessor. You can create websites, android app or iOS app. PHP is platform independent. It supports a large range of database.