dropdownlist1.Items.Insert(0, new ListItem("None", "0"));
for validating you can use javascript for example
try
{
if(dropdownlist1.SelectedValue == "0")
{
throw new Exception("<script language="javascript">" + "alert('Please select something');" + "");
}
}
catch (Exception ex)
{
label1.Text = ex.Message;
}</script>