try this, for asp
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function userValid() {
var Description = document.getElementById("<%= TxtDescription.ClientID %>");
if (Description.value == "") {
alert("Please Mention at Description");
Description.focus();
return false;
}
return true;
}
</script>
</head>
<body>
<form id="frm" runat="server">
<asp:TextBox runat="server" ID="TxtDescription" />
<asp:Button Text="Save" OnClientClick="return userValid();" runat="server" />
</form>
</body>
</html>