fai cosi:
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Checkbox</title>
<script type="text/javascript">
function controllo(){
if (document.getElementById('pasta').checked || document.getElementById('riso').checked || document.getElementById('lasagne').checked) {
alert("ORDINE EFFETTUATO");
}else{alert("ESEGUIRE UNA SCELTA");
return false;}
}
</script>
</head>
<body>
<form method="get" name="menu" onsubmit ="return controllo()">
<input type="checkbox" name="pasta" id="pasta" />Pasta
<input type="checkbox" name="riso" id="riso" />Riso
<input type="checkbox" name="lasagne" id="lasagne" />Lasagne
<input type="submit" value="Ordina"/>
</form>
</body>
</html>
Consiglio di leggere qui