initally set the AutoPostBack true for the DropDownList and 
in SelectedIndexChaged() method of the DropDrownList write 
the business logig for the details to be displayed..

You have to set the Autopostback property of dropdown to 
true. And in the dropdownlist's selecetedIndexChanged() 
event write the logic to get the value to testbox.

 For eg:
private void cmbOperationSelect_SelectedIndexChanged(object 
sender, EventArgs e)
        {
            textbox1.Text = 
cmbOperationSelect.Selecteditem.value;
}