Puoi fare cosi.... per il lato client, ma devi anche fare il controllo lato server.
codice:
<script language="JavaScript" type="text/javascript">
<!--
function controlla() {
if (document.mioform.select1.value == "Scegli") {
alert('campo obbligatorio');
return false;
}
return true;
}
//-->
</script>
...
<form name="mioform" action="pippo.php" method="post" onsubmit="return controlla()">
...
<select name="select1">
<option selected>Scegli</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
<input type="submit" value="invia">
</form>