[eluser]vitoco[/eluser]
you must attach the "onchange" event to the states select , then you have 2 options ,
1.- load the city list with ajax. sending the selected state value and retrieving only the cities that belong to that state
2.- post the form to the server, get the value of the states select and populate the city select according to that value and reaload the page ( actually sending the whole page again to the browser )
Saludos