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