Jim, not entirely correct.
You can add a listitem with the overloaded .Add() method of the ListItemCollection class, but you have to provide it a string that will serve as text and value for the item:
ddlid.Items.Add(x.ToString());
Muskaan, please not that your loop will never be entered into. The testlogic is backwards:
for(x=1; x>=12; x++)
should be
for(x=1; x<=12; x++)
-
Peter