Take a jquery.js file.
<html>
<head>
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$(btnClick).click(function(){
alert("Hello! You have clicked me.");
});
});
</script>
</head>
<body>
<input type="button" id="btnClick" value="Click" />
</body>
</html>