Devi usare l'evento onsubmit sul form. Se la funzione ritorna false allora il submit non viene eseguito altrimenti se ritorna true il submit viene eseguito.
Per il controllo del campo mail, troverai cio' di cui hai bisogno cercando nel forum.
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function controlla() {
if (document.modulo.nome.value == '') {
alert('nome obbligatorio');
return false;
}
//
if (document.modulo.cognome.value == '') {
alert('cognome obbligatorio');
return false;
}
return true;
}
//-->
</script>
</head>
<body>
<form action="registra.php" method="post" name="modulo" onsubmit="return controlla()">
<input name="idstrut" type="hidden" id="idstrut" value="1">
<table border="0" align="center" cellpadding="3" cellspacing="0" bgcolor="#EBEBEB">
<tr>
<td width="104" height="26" bgcolor="#FFFFFF" >
<div align="right"> <font color="#000000">Nome </font></div></td>
<td width="12" height="26" bgcolor="#FFFFFF" >
<div align="center">- </div></td>
<td width="202" bgcolor="#FFFFFF" >
<div align="left">
<input name="nome" type="text" id="nome">
</div></td>
</tr>
<tr>
<td width="104" height="2" bgcolor="#FFFFFF" >
<div align="right"> Cognome</div></td>
<td width="12" height="2" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td width="202" bgcolor="#FFFFFF">
<div align="left">
<input name="cognome" type="text" id="cognome">
</div></td>
</tr>
<tr>
<td height="2" bgcolor="#FFFFFF">
<div align="right"><font color="#000000">Indirizzo</font></div></td>
<td height="2" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td bgcolor="#FFFFFF">
<div align="left">
<input name="indirizzo" type="text" id="indirizzo">
</div></td>
</tr>
<tr>
<td height="2" bgcolor="#FFFFFF">
<div align="right"><font color="#000000">Città</font></div></td>
<td height="2" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td bgcolor="#FFFFFF">
<div align="left">
<input name="citta" type="password" id="citta">
</div></td>
</tr>
<tr>
<td height="26" bgcolor="#FFFFFF">
<div align="right"><font color="#000000"> Telefono </font></div></td>
<td height="26" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td bgcolor="#FFFFFF">
<div align="left">
<input name="telefono" type="text" id="telefono">
</div></td>
</tr>
<tr>
<td height="26" bgcolor="#FFFFFF">
<div align="right"><font color="#000000"> Note:</font></div></td>
<td height="26" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td bgcolor="#FFFFFF">
<div align="left">
<textarea name="textarea" id="textarea"></textarea>
</div></td>
</tr>
<tr>
<td height="26" bgcolor="#FFFFFF">
<div align="right"><font color="#000000"> E-mail</font></div></td>
<td height="26" bgcolor="#FFFFFF">
<div align="center">- </div></td>
<td bgcolor="#FFFFFF">
<div align="left">
<input name="mail" type="text" id="mail">
</div></td>
</tr>
</table>
<table width="242" border="0" cellspacing="0" cellpadding="0" align="center">
<tr>
<td height="2">
<div align="center">
<input type="submit" name="Submit" value="Invia" style="color: #000000; font-family: Arial; font-size: 8 pt; background-color: #cc6600 ">
<input type="reset" name="Reset" value="reset" style="color: #000000; font-family: Arial; font-size: 8 pt; background-color: #cc4400">
</div></td>
</tr>
</table>
</form>
</body>
</html>