Use the script correctly,
Try this script, for each textbox,dropdown and other controls.
<script language="javascript" type="text/javascript">
function Validate() {
if (document.getElementById('<%= dropdownid.ClientID%>').value == "Select") {
alert("Please Select Details!");
document.getElementById('<%= dropdownid.ClientID%>').focus();
return false;
}
if (document.getElementById('<%=textboxId.ClientID %>').value == "") {
alert("Please Enter Your Text point!");
document.getElementById('<%=textboxId.ClientID %>').focus();
return false;
}
}
</script>
call the validate function in your submit button,
for example,
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick ="return Validate();" />