Before the closing body tag add this (reference to jQuery library). Other hosted libraries can be found here
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
And this
<script>
//paste your code here
</script>
It should look something like this
<body>
........
........
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
<script> Your code </script>
</body>