I Need Ideal On Coding a WordPress Plugin

Am a beginner in wp coding and some times I wonder how other who builds there’s does so, I wish to build a plugin by my self of a special function, and a short code.

Example : a plugin that adds subscribe to our feed option below a post, or widget subscriber option. Or better still a social share button below a post.

I also wish to create related post plugin in which users could use short code to add a set of related in a wp content.

The only knowledge I have as per this is the following

<? php
Plugin Description: all about the plugin
Pluign author: author name
Author url: url of the author
//// comment ////

Then the main code and functions here
}
?>

Am not new to HMTL Tags and PHP coding but actually this code style is very different.

I seriously need help on putting in a function, and calling the function out.

Thanks in advance