I want to know what part I should study to make a program.

I will be discharged from the military in November this year and return to school in March next year, but I have a question because I want to study before returning to school. When I was a freshman, I didn't even know the language of programming and started the semester, and people from clubs and departments gathered together It was a pity that I couldn't participate in various projects or competitions. So, I want to return to school after accumulating some knowledge and skills, so I am studying Java using Java books, programmers, and cloud IDE in private areas. As I studied a little bit, it was vague, and I thought that I would choose a program that I wanted to make and study necessary because I saw a lot of advice that it would be better to set a program or goal and study. I want to create an app that contains information similar to the "Roll Encyclopedia" application and allows people to post in addition to it I wonder what kind of study I need.

And I wonder how or what other studies should be done to make Java's coded program run in the form of a Windows wallpaper or icon instead of Eclipse and easy to run in the form that people commonly use.