function checkForm() {
var firstPasswordField=document.nomeform.password01.value;
var secondPasswordField=document.nomeform.password02.value;

if (firstPasswordField!=secondPasswordField) {
 alert("ATTENZIONE!\nLe due password non coincidono!);
}
}

....
<form name="nomeform">
<input type="password" name=firstPasswordField"" value="">
<input type="password" name="secondPasswordField" value="">
</form>