jQuery documentation says you can call the datepicker by this command:
$("#datepicker").datepicker();
If you click the 'view source' button on the documentation page you can see that they've wrapped it into the ready function:
$(document).ready(function(){
$("#datepicker").datepicker();
});
EDIT: It should work with INPUT (thanks for pointing this out Steerpike). This is the test I've written and it works, try it yourself:
<html>
<head>
<link type="text/css" href="http://jqueryui.com/latest/themes/base/ui.all.css" rel="stylesheet" />
<script type="text/javascript" src="http://jqueryui.com/latest/jquery-1.3.2.js"></script>
<script type="text/javascript" src="http://jqueryui.com/latest/ui/ui.core.js"></script>
<script type="text/javascript" src="http://jqueryui.com/latest/ui/ui.datepicker.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("#datepicker").datepicker();
});
</script>
</head>
<body>
<input type="text" id="datepicker" value="this is a test">
</body>
</html>