listbox... where is "onclick" ? ? ?

Please, help-.....on ASP net, how can i control the listbox click ?

I want to do this :

I have a list box full of registers comming from a Database. Then the user can click on any register and i want to show hiim more onformation about the resgister he has clicked....


With classical list box and Javascript :

<select name="listAltes" size="20" ondclick="MostrarVentana1()">


any possibilities ? ?
Thank you in advance !

on the client use:
myList.Attributes.Add("onclick","somefunction");

on the server use SelectedIndexChanged event - and set AutoPostBack to true on the control.