Theme needs fixing for plugins to work

Hi there,

I am new to website design and building and am having troubles achieving the design that I want. The top of my admin page says:

“Your theme needs to be fixed for plugins to work. To fix your theme, use the Theme Editor to insert <?php wp_head(); ?> just before the </head> line of your theme’s header.php file.”

Can anyone tell me what this means? I have tried doing what it says but the header.php has nothing in it. It reads:

<?php

/*
Empty on purpose.
*/

Thanks for the help,