codice:
<slect id="film" size=1 name="film">
<option>scegli un film</otpion>
<option value="gla">il gladiatore</option>
<option value="ara">arancia meccanica</option>
<option value="ftz">fantozzi</option>
</select>

<script>
function verFilm()
{
var MySel=document.getElementById('film');
if (MySel.SelectedIndex == 0)
{
alert('prego selezionare un film');
return false;
}
return true
}
</script>

<form ... onSubmit="return verFilm()">