<script lang="javascript" type="text/javascript">
function Validation() {
var name = document.getElementById('<%=txtFirstName.ClientID %>');
var fathername = document.getElementById('<%=txtFatherName.ClientID %>');
var address = document.getElementById('<%=txtAddress.ClientID %>');
if (name.value == "") {
alert("Please Enter First Name");
name.focus();
return false;
}
if (fathername .value == "") {
alert("Please Enter father Name");
fathername .focus();
return false;
}
if (address.value == "") {
alert("Please Enter Address");
address.focus();
return false;
}
}
</script>
and call function as below
<asp:button id="btn" runat="server" OnClientClick="javascript:return Validation()" />
If you are using html button then you need to call the javascript function in onclick event.