This is my code, I need help validating the fields that the users types in. If anyone could help me out, I would appreciate it very much.

<html>
<head>

<script src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript" src="http://jzaefferer.github.com/jquery-validation/jquery.validate.js"></script>


<script>
$(document).ready(function(){
$("#commentForm").validate();
});
</script>
<script>

var lastNames= [];
var firstNames= [];
var percent = [];
var grade = [];


function addToArray() {

var ln = document.getElementById('lastName');
var fn = document.getElementById('firstName');
var gr = document.getElementById('percent');
var per = document.getElementById('grade');

lastNames[lastNames.length] = ln.value;
firstNames[firstNames.length] = fn.value;
percent[percent.length] = per.value;
grade[grade.length] = gr.value;



alert( lastNames );
alert( firstNames );

alert( percent );
alert( grade );

}

</script>
</head>
<body>
<form class="entry" id="enter" method="get" action="">
Enter last name: <input type="text" id="lastName"><br>
Enter first name: <input type="text" id="firstName"><br>
Enter percent: <input type="text" id="percent"><br>
Enter grade : <input type="text" id="grade"><br>
<input type="button" value="Submit" onClick="addToArray()">
</body>
</html>