spiegati meglio:

codice:
<html>
<head>
<script language="javascript">

function selectDeal() {
  mySel = document.getElementById("citta");
  if(mySel.selectedIndex == 0) {
    alert("Devi scegliere un valore");
  }
  else {
    alert("Hai scelto: "+mySel.value+"\n\n"+"Di indice: "+mySel.selectedIndex);
  }
}
</script>
</head>
<body>
<select name="citta" id="citta">
<option value="" selected>---Scegliere---</option>
<option value="Venezia">Venezia</option>
<option value="Roma">Roma</option>
<option value="Napoli">Napoli</option>
<option value="Catania">Catania</option>
</select><input type="button" value="Clicka e Godi" onClick="selectDeal();">
</body>
</html>
questo va