Here is one place you can start if you are looking for basic info on creating a plugin:
http://codex.wordpress.org/Writing_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:
<?php
/*
Plugin Name: My Plugin
Plugin URI: http://blog.kennyray.com
Description: This is a plugin
Version: 1.0
Author: Kenny Ray
Author URI: http://blog.kennyray.com
License: GPL2
*/
?>
At this point, you will see your Plugin in the plugins section of the site admin.