It sounds like you need an introduction to plugin development. As in all things WordPress Development, the WordPress Codex is the first place to look.
Here is the Plugin Development information: https://developer.wordpress.org/plugins/
But you will need some PHP programming knowledge. Here’s one good place to start: https://www.w3schools.com/php/default.asp . And the googles have plenty of information about PHP programming and tips and sample source code.
You should develop on a non-vital site – could be a testing site on your local computer, or a testing area with a WP install on your hosting site.
Good luck!