What hook to use?

Question

WordPress

I need to write a plugin, upon activation of which, a cryptographic will be placed on the page. I wrote cryptographs, but I have never created plugins and it doesn’t work with PHP. What will this plugin look like? That's all I could understand

if(!defined('ABSPATH')){ //Защита от злоумышлинников
     die; 
 }

 class JapCanChart
 {
     //method
     function __construct(){

    }
 }
if(class_exists('JapCanChart')){
    new JapCanChart();

}

But what to do next, I absolutely do not understand.
My code consists of HTML (and inside the html is css and js (just such a construction is needed))