dropdownlist validation in web form ???

Hi, I'm new in C# (two weeks) and I need to control a dropdownlist with some validator, I've used a required field validator with initial value="", and a regularexpresion validator with myddl.SelectedValue!="" but none of them works. The list has an item with value "" just to start with it, with nothing, but the user must select one element. How can I control this? Thanx in advance.