How do I create a simple joomla plugin?

I'm having a real problem unstanding somthing thats probably very easy about creating and using joomla plugins.

Here is what I've done so far.

I've created a sample joomla plugin using the following two files inside of a folder and named them all the same.

I listed their contents below.

The plugin installs correctly through the admin panel

Then I enable it through plugin manager

ok. all set to go.

How do I use the plugin on an article once I've enabled the plugin?

ZIP FOLDER: MakePlugIn FOLDER: MakePlugIn

MakePlugIn.php -

<?php 
// No direct access allowed to this file
defined( '_JEXEC' ) or die( 'Restricted access' );

// Import Joomla! Plugin library file
jimport('joomla.plugin.plugin');

//The Content plugin MakePlugIn
class plgContentMakePlugIn extends JPlugin
{
    function plgContentMakePlugIn (&$subject)
    {
        parent::__construct ($subject);
    }
    function onPrepareContent (&$article, &$params, $page=0)
    {
        print "I am a happy plugin";
    }
}
?>

MakePlugIn.xml -

<?xml version="1.0" encoding="utf-8"?>
<install version="1.5" type="plugin" group="content">
    <name>Make-Plug-In</name>
    <author>Make-Plug-In</author>
    <creationDate>03/15/2011</creationDate>
    <copyright>Copyright (C) 2011 Holder. All rights reserved.</copyright>
    <license>GNU General Public License</license>
    <authorEmail>authoremail@website.开发者_StackOverflow中文版com</authorEmail>
    <authorUrl>www.authorwebsite.com</authorUrl>
    <version>1.0</version>
    <description>Make-Plug-In test</description>
    <files>
        <filename plugin="MakePlugIn">MakePlugIn.php</filename>
    </files>
</install> 

You should not be echoing or printing information in the plug-in.

The method is receiving article reference as a parameter, modify it and you are good. You can use var_dump to quickly identify proper object type and properties.

Here is Joomla tutorial on creating Content Plug-in.

Updated on 3/17/2011

This is in response to first comment. In order to modify the article modify the value of referenced object &$article. See example below:

function onPrepareContent( &$article, &$params, $limitstart )
{
    //   Include you file with ajax code
    JHTML::_('script', 'ajax-file.js', 'media/path/to/js/dir/');

    //   Create ajax div
    $ajaxDiv = '<div id="ajax-div"></div>';

    // Modify article text by adding the div for ajax at the top
    $article->text = $ajaxDiv . PHP_EOL . $article->text;

    return true;
}

Adding external JS to the head of the document.