After your DataBind():
lstDepartment.SelectedIndex = 0; //first item
or
lstDepartment.SelectedValue = "Yourvalue"
or
//add error checking, just an example, FindByValue may return null
lstDepartment.Items.FindByValue("Yourvalue").Selected = true;
or
//add error checking, just an example, FindByText may return null
lstDepartment.Items.FindByText("Yourvalue").Selected = true;