The minimum code for the xml file is the tag <extension> with the attributes type="module" version="3.0" cllient="site" method="upgrade"> and within the extension tag is the tags <name> <description> <files> and within the files tag the <filename> tags destined each file in the module. The main php file will have an attribute module = and then the name of the module. Is this case mod_foo.php
Code this in your mod_foo.xml file
<?xml version="1.0" encoding="utf-8"?>
<extension
type="module"
version="3.0"
cllient="site"
method="upgrade">
<name>Foo Module</name>
<description>My first module</description>
<files>
<filename module="mod_foo">mod_foo.php</filename>
<filename>index.html</filename>
</files>
</extension>
Chapter 4 - Install to Joomla Extension Manager Module Development