How to make a program for Linux

Answer 1

The most used programming language in Linux is C++. It has Object Oriented capabilities and can be used with Graphic Interface Libraries like GTk and Qt.
The second most used language is Java. That is totally Object Oriented and can be run in practically all platform included Android, Windows, OSX, etc.

Answer 2

The second one most used language in Linux (at least Debian branch, which is the major Linux branch) is rather Python than Java ^^

Answer 3

You can create programs for Linux in the same way any other plataform, you can wirte programs in C, C++, Java, Python, Ruby, JavaScript (NodeJS), Golang, or whatever programming language you want, and compile it or run it with the appropiate interpreter.

If you do not know any programming languages yet, start with Python..