Here is one place you can start if you are looking for basic info on creating a plugin:
Create a file in the plugins folder. Call it whatever you’d like.
Place a comment block at the top like this, filling in the info specific to your plugin:
Plugin Name: My Plugin
Plugin URI: http://blog.kennyray.com
Description: This is a plugin
Author: Kenny Ray
Author URI: http://blog.kennyray.com
At this point, you will see your Plugin in the plugins section of the site admin.