Use the below code, this may help you:
<table>
<tr id="trReportAgency" runat="server">
<td class="fieldlabel">
<asp:Label ID="lblAgency" runat="server" Text="Agency:" />
</td>
<td>
<asp:DropDownList ID="ddlAgency" runat="server" CssClass="listbox" Width="138px" >
<asp:ListItem Text="Select" Value="-1"></asp:ListItem>
<asp:ListItem Text="1" Value="1"></asp:ListItem>
<asp:ListItem Text="2" Value="2"></asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="rqAgency" runat="server" ControlToValidate="ddlAgency"
CssClass="validatorIndicator" Display="Dynamic" ErrorMessage="Please select agency."
InitialValue="-1" Text="Please select agency" ToolTip="Please select agency." />
<asp:Button ID="btnPrint" runat="server" Text="Print" OnClick="btnPrint_Click" />
</td>
</tr>
</table>
Thanks,