Creating a helper is quite easy, go on your /etc module directory and edit your config.xml file
ex : app/code/local/Adin/Epub/etc/config.xml
Add a block inside the <global> and after </blocks>

<?xml version="1.0"?>
<config>
	<modules>
		<Adin_Epub>
			<version>1.0.0</version>
		</Adin_Epub>
	</modules>
	
	<global>
		<helpers>
			<epub>
				<class>Adin_Epub_Helper</class>
			</epub>
		</helpers>
	</global>
	
</config>

Then creates the Helper folder on your module directory and create a data class :
app/code/local/Adin/Epub/Helper/Data.php

<?php
class Adin_Epub_Helper_Data extends Mage_Core_Helper_Abstract
{
    //fonction de test
    public function getTest()
    {
        return "Ceci est un test de helper";
    }	
}

If you create a new module, don’t forget to activate it, for that, in app/etc/modules directory, create or append a xml file and add your new module, for example :

<?xml version="1.0"?>
<config>
    <modules>
        <Adin_Epub>
            <active>true</active>
            <codePool>local</codePool>
        </Adin_Epub>
    </modules>
</config>

Now, for using your new helper, just instanciate it and call the function :

$helper = Mage::helper('epub');
echo $helper->getTest();