<div>
<asp:DropDownList ID="dl" runat="server">
<asp:ListItem Value="0">Select One Name</asp:ListItem>
<asp:ListItem>Sandeep</asp:ListItem>
<asp:ListItem>Pradeep</asp:ListItem>
<asp:ListItem>Dinesh</asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="rf" runat="server" ErrorMessage="Select One Name" ControlToValidate="dl" InitialValue="0"></asp:RequiredFieldValidator>
</div>
<div><asp:Button ID="btn" runat="server" Text="submit" /></div>