Magento – How to add new custom field into customer form and then save it into database in Magento

The customer data is based on EAV, this means you'll need to add an attribute to the attribute set instead of a table column.

There are a couple of tutorials on that, check out this one and this one. It comes down to creating your own module with an install script that has something like this

$installer = $this;
$installer->startSetup();

$installer->addAttribute('customer', 'temp_id', array(
    'type' => 'varchar',
    'input' => 'text',
    'label' => 'Temporary ID',
    'global' => 1,
    'visible' => 1,
    'required' => 0,
    'user_defined' => 0,
    'default' => '',
    'visible_on_front' => 0,
    'source' => NULL,
));

$installer->endSetup();

config.xml

<default>
    ...
    <resources>
        <your_module_identifier>
            <setup>
                <module>Your_ModuleName</module>
                <class>Mage_Customer_Model_Resource_Setup</class>
            </setup>
        </your_module_identifier >
    </resources>
    ...
</default>

Ok. Lets Go to your "billing.phtml" file Which is Located at

frontend\rwd\default\template\persistent/checkout/onepage/billing.phtml

And Paste Following Code If You want to Checkbox Field.

<div class="field">
    <input id="p_method_cashondelivery" value="cashondelivery" type="checkbox" name="cod" title="Cash On Delivery" onclick="<?php echo "checked!!!!";?>" class="checkbox" autocomplete="off">
    <label for="p_method_cashondelivery">Cash On Delivery </label>
</div>

Same Way You Can Add Custom Field In Shipping Address File.

frontend\rwd\default\template\checkout/onepage/shipping.phtml