<!--add to your html between the head tags-->
<script type="text/javascript">
function validate ( )
{
	valid = true;
	if ( document.contact_form.first_name.value == "" )
	{
		alert ( "Please enter your first name." );
		document.contact_form.first_name.focus();
		valid = false;
	}
	return valid;
}
</script>


<!--add to your html-->
<form action="#" name="contact_form" class="above" method="post" onsubmit="return validate( );">
<fieldset>
<legend>Contact form</legend>
<label for="first">First</label><input type="text" name="first_name" id="first_name">
<label for="last">Last</label><input type="text" name="last_name" id="last_name">
<label for="email">Email</label><input type="text" name="email" id="email">
<label for="message">Message</label><textarea name="message" rows="10" cols="20"></textarea>
<br />
<input type="submit">
</fieldset>
</form>

Rendered

Contact form