Comb box issue

Question

Hi i all i have my combobox as follows i have initially assign the text property to Select.
 And in item collection i have some values as PPD,CCD.

Now i am checking for the initial value like

if (comobox1.SelectedText!="Select")
{
   do some thing
}
else

{

    i will have a tooltip to raise an error for this as tooptip.show("req",combobox1);
}

What i need is initiallt combo box should have select if the user does not select other option from combo box it should raise an error .. Any Help