How to apply Requiredfiel Validator in dropdown

Hello friends,
i want to apply require field validator in dropdown which i given in grid view item tamplate. i tried like below but not working.
plz suggest me.
<asp:TemplateField HeaderText="Courier Name" SortExpression="CourierName"> <EditItemTemplate> <asp:DropDownList ID="ddlCouriername" runat="server"> <asp:ListItem Value="0">Select Courier</asp:ListItem> <asp:ListItem Value="1">DTDC</asp:ListItem> <asp:ListItem Value="2">Blue Dart</asp:ListItem> <asp:ListItem Value="3">Blaze Flash</asp:ListItem> <asp:ListItem Value="4">Speed Post</asp:ListItem> ...