You have to give jquery file reference.
Try this updated code...It will work..Enjoy!! :)
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>
<title></title>
<script type='text/javascript'>
$(document).ready(function () {
$("#myhref").on("click", function (e) {
alert("Hello world - Alert");
document.write('Hello World!');
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
Welcome to my Site..
<a href="#" id="myhref">Add to cart</a>
</div>
</form>
</body>
</html>