Put the sub menu items into a separate ul, and use jQuery to toggle this ul on click event. E.g.
<!DOCTYPE html>
<html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
$(".aboutus").click(function(){
$(".aboutusitem").toggle();
});
});
</script>
</head>
<body>
<ul class="menu">
<li>Others</li>
<li class="aboutus">ABOUT US</li>
<ul class="aboutusitem">
<li><a href="contact.html">CONTACT</a></li>
<li><a href="http://www.jcythailand.com/jcy" target = "blank">Service</a> </li>
</ul>
</il>
</ul>
</body>
</html>
Read more:
jQuery toggle() Method[
^]