I am quite new to wordpress, And quite confused about wordpress.

I am building a website to write programming topics. When user is suppose opened "Java Inheritance in the left sidebar must show the list of all the topics covered in Java.

For example, I took inspiration from this site. How to show the topics list in left sidebar.

I want to make php file, which can be installed in wordpress folder inside server. And how can I proceed to write the PHP and what to write in the PHP file which should show topics of Java in sidebar.

Correct my errors.Thanks