You must include the library jQueryin the section <head></head>on the page:
<script language="javascript" src="http://code.jquery.com/jquery.min.js"></script>
And file semantic.js:
<script language="javascript" src="[your path here]/semantic.js"></script>
What you need to download from:
http://semantic-ui.com/
( dist. src )
BETWEEN <head></head>:
<script language="javascript" src="http://code.jquery.com/jquery.min.js"></script>
<script language="javascript" src="[your path here]/semantic.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$('.left.demo.sidebar').sidebar('toggle');
});
</script>