How to validate a DropDownList box in ASP.NET?

You can use RequiredFieldValidator control with InitialValue as the default value for the DropDownList box.

<asp:DropDownList id="DropDownList1" runat="server">
   <asp:ListItem Selected="True">Select a name</asp:ListItem>
   <asp:ListItem>Ram</asp:ListItem>
   <asp:ListItem>Shyam</asp:ListItem>
   <asp:ListItem>Mohan</asp:ListItem>
</asp:DropDownList>

<asp:RequiredFieldValidator id="RequiredFieldValidator1"  
  runat="server" ErrorMessage="Please make a selection" 
  ControlToValidate="DropDownList1" 
  InitialValue="Select a name" >
</asp:RequiredFieldValidator>

In this case, unless the user will select other item than "Select a name" from the dropdown, the form will not be submitted.