Plugin is an extension which provides a function that is associated with any trigger events. Following are the files required for creating a simple custom plugin in Joomla.

1 myfirsttest.xml
2 myfirsttest.php
3 index.html

First for creating a custom plugin we need to create an xml file which includes information about the author of plugin and what files are required for that plugin.

<extension version="2.5" type="plugin" group="content">
        <name>plg_content_myfirsttest</name>
        <author>Kshitiz dixit</author>
        <creationdate>december 18th, 2015</creationDate>   
    <copyright>Evontech</copyright>
        <license>GNU General Public License</license>
        <authoremail>kshitiz.dixit@evontech.com</authoremail>
        <authorurl>http://www.evontech.com</authorurl>
        <version>1.0</version>
        <description>Simple custom plugin which prints text  in any article content.</description>
        <files>
                <filename plugin="myfirsttest">myfirsttest.php</filename>
                <filename>index.html</filename>
        </files>
</creationdate></extension>

Next step is to create a php file which does all the work. As we are creating a content based plugin name my firsttest, so file should be saved as myfirsttest.php.

<?php

defined('__JEXEC') or die;

class plgContentMyfirsttest extends JPlugin
{
    public function onContentAfterTitle($context, &amp;$article, &amp;$params, $limitstart)
        {
                return "my first test plugin created successfully!";
        }
}

?>

Next and last step is to create an index.html file in which we don't need to place any code inside that file and compress into a zip file name as myfirsttest.zip.