form validation

As Evil Phil says, server side is the place to do it. Doing it client side should only be for the purposes of usability, and not relied on by the site.

That said, here's a quick example you can work from:
[html]
<script type="text/JavaScript">
function checkForm()
{
if (document.getElementById("txtName").value == "")
{
alert("Please enter your name");
return false;
}
return true;
}
</script>
<form name="frmTest" action="this.asp" method="post" onsubmit="return checkForm()">
Name: <input type="text" name="txtName" id="txtName" value="" />
<input type="submit">
</form>
[/html]